Add scripting (tengo) support for every incoming message (#731)

TengoModifyMessage allows you to specify the location of a tengo (https://github.com/d5/tengo/) script.
This script will receive every incoming message and can be used to modify the Username and the Text of that message.
The script will have the following global variables:
to modify: msgUsername and msgText
to read: msgChannel and msgAccount

The script is reloaded on every message, so you can modify the script on the fly.

Example script can be found in https://github.com/42wim/matterbridge/tree/master/gateway/bench.tengo
and https://github.com/42wim/matterbridge/tree/master/contrib/example.tengo

The example below will check if the text contains blah and if so, it'll replace the text and the username of that message.
text := import("text")
if text.re_match("blah",msgText) {
    msgText="replaced by this"
    msgUsername="fakeuser"
}

More information about tengo on: https://github.com/d5/tengo/blob/master/docs/tutorial.md and
https://github.com/d5/tengo/blob/master/docs/stdlib.md

vendor/github.com/d5/tengo/compiler/token/keywords.go

@@ -0,0 +1,19 @@
+package token
+
+var keywords map[string]Token
+
+func init() {
+	keywords = make(map[string]Token)
+	for i := _keywordBeg + 1; i < _keywordEnd; i++ {
+		keywords[tokens[i]] = i
+	}
+}
+
+// Lookup returns corresponding keyword if ident is a keyword.
+func Lookup(ident string) Token {
+	if tok, isKeyword := keywords[ident]; isKeyword {
+		return tok
+	}
+
+	return Ident
+}

vendor/github.com/d5/tengo/compiler/token/tokens.go

@@ -0,0 +1,208 @@
+package token
+
+import "strconv"
+
+// Token represents a token.
+type Token int
+
+// List of tokens
+const (
+	Illegal Token = iota
+	EOF
+	Comment
+	_literalBeg
+	Ident
+	Int
+	Float
+	Char
+	String
+	_literalEnd
+	_operatorBeg
+	Add          // +
+	Sub          // -
+	Mul          // *
+	Quo          // /
+	Rem          // %
+	And          // &
+	Or           // |
+	Xor          // ^
+	Shl          // <<
+	Shr          // >>
+	AndNot       // &^
+	AddAssign    // +=
+	SubAssign    // -=
+	MulAssign    // *=
+	QuoAssign    // /=
+	RemAssign    // %=
+	AndAssign    // &=
+	OrAssign     // |=
+	XorAssign    // ^=
+	ShlAssign    // <<=
+	ShrAssign    // >>=
+	AndNotAssign // &^=
+	LAnd         // &&
+	LOr          // ||
+	Inc          // ++
+	Dec          // --
+	Equal        // ==
+	Less         // <
+	Greater      // >
+	Assign       // =
+	Not          // !
+	NotEqual     // !=
+	LessEq       // <=
+	GreaterEq    // >=
+	Define       // :=
+	Ellipsis     // ...
+	LParen       // (
+	LBrack       // [
+	LBrace       // {
+	Comma        // ,
+	Period       // .
+	RParen       // )
+	RBrack       // ]
+	RBrace       // }
+	Semicolon    // ;
+	Colon        // :
+	Question     // ?
+	_operatorEnd
+	_keywordBeg
+	Break
+	Continue
+	Else
+	For
+	Func
+	Error
+	Immutable
+	If
+	Return
+	Export
+	True
+	False
+	In
+	Undefined
+	Import
+	_keywordEnd
+)
+
+var tokens = [...]string{
+	Illegal:      "ILLEGAL",
+	EOF:          "EOF",
+	Comment:      "COMMENT",
+	Ident:        "IDENT",
+	Int:          "INT",
+	Float:        "FLOAT",
+	Char:         "CHAR",
+	String:       "STRING",
+	Add:          "+",
+	Sub:          "-",
+	Mul:          "*",
+	Quo:          "/",
+	Rem:          "%",
+	And:          "&",
+	Or:           "|",
+	Xor:          "^",
+	Shl:          "<<",
+	Shr:          ">>",
+	AndNot:       "&^",
+	AddAssign:    "+=",
+	SubAssign:    "-=",
+	MulAssign:    "*=",
+	QuoAssign:    "/=",
+	RemAssign:    "%=",
+	AndAssign:    "&=",
+	OrAssign:     "|=",
+	XorAssign:    "^=",
+	ShlAssign:    "<<=",
+	ShrAssign:    ">>=",
+	AndNotAssign: "&^=",
+	LAnd:         "&&",
+	LOr:          "||",
+	Inc:          "++",
+	Dec:          "--",
+	Equal:        "==",
+	Less:         "<",
+	Greater:      ">",
+	Assign:       "=",
+	Not:          "!",
+	NotEqual:     "!=",
+	LessEq:       "<=",
+	GreaterEq:    ">=",
+	Define:       ":=",
+	Ellipsis:     "...",
+	LParen:       "(",
+	LBrack:       "[",
+	LBrace:       "{",
+	Comma:        ",",
+	Period:       ".",
+	RParen:       ")",
+	RBrack:       "]",
+	RBrace:       "}",
+	Semicolon:    ";",
+	Colon:        ":",
+	Question:     "?",
+	Break:        "break",
+	Continue:     "continue",
+	Else:         "else",
+	For:          "for",
+	Func:         "func",
+	Error:        "error",
+	Immutable:    "immutable",
+	If:           "if",
+	Return:       "return",
+	Export:       "export",
+	True:         "true",
+	False:        "false",
+	In:           "in",
+	Undefined:    "undefined",
+	Import:       "import",
+}
+
+func (tok Token) String() string {
+	s := ""
+
+	if 0 <= tok && tok < Token(len(tokens)) {
+		s = tokens[tok]
+	}
+
+	if s == "" {
+		s = "token(" + strconv.Itoa(int(tok)) + ")"
+	}
+
+	return s
+}
+
+// LowestPrec represents lowest operator precedence.
+const LowestPrec = 0
+
+// Precedence returns the precedence for the operator token.
+func (tok Token) Precedence() int {
+	switch tok {
+	case LOr:
+		return 1
+	case LAnd:
+		return 2
+	case Equal, NotEqual, Less, LessEq, Greater, GreaterEq:
+		return 3
+	case Add, Sub, Or, Xor:
+		return 4
+	case Mul, Quo, Rem, Shl, Shr, And, AndNot:
+		return 5
+	}
+	return LowestPrec
+}
+
+// IsLiteral returns true if the token is a literal.
+func (tok Token) IsLiteral() bool {
+	return _literalBeg < tok && tok < _literalEnd
+}
+
+// IsOperator returns true if the token is an operator.
+func (tok Token) IsOperator() bool {
+	return _operatorBeg < tok && tok < _operatorEnd
+}
+
+// IsKeyword returns true if the token is a keyword.
+func (tok Token) IsKeyword() bool {
+	return _keywordBeg < tok && tok < _keywordEnd
+}