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/parser/error.go

@@ -0,0 +1,21 @@
+package parser
+
+import (
+	"fmt"
+
+	"github.com/d5/tengo/compiler/source"
+)
+
+// Error represents a parser error.
+type Error struct {
+	Pos source.FilePos
+	Msg string
+}
+
+func (e Error) Error() string {
+	if e.Pos.Filename != "" || e.Pos.IsValid() {
+		return fmt.Sprintf("Parse Error: %s\n\tat %s", e.Msg, e.Pos)
+	}
+
+	return fmt.Sprintf("Parse Error: %s", e.Msg)
+}

vendor/github.com/d5/tengo/compiler/parser/error_list.go

@@ -0,0 +1,68 @@
+package parser
+
+import (
+	"fmt"
+	"sort"
+
+	"github.com/d5/tengo/compiler/source"
+)
+
+// ErrorList is a collection of parser errors.
+type ErrorList []*Error
+
+// Add adds a new parser error to the collection.
+func (p *ErrorList) Add(pos source.FilePos, msg string) {
+	*p = append(*p, &Error{pos, msg})
+}
+
+// Len returns the number of elements in the collection.
+func (p ErrorList) Len() int {
+	return len(p)
+}
+
+func (p ErrorList) Swap(i, j int) {
+	p[i], p[j] = p[j], p[i]
+}
+
+func (p ErrorList) Less(i, j int) bool {
+	e := &p[i].Pos
+	f := &p[j].Pos
+
+	if e.Filename != f.Filename {
+		return e.Filename < f.Filename
+	}
+
+	if e.Line != f.Line {
+		return e.Line < f.Line
+	}
+
+	if e.Column != f.Column {
+		return e.Column < f.Column
+	}
+
+	return p[i].Msg < p[j].Msg
+}
+
+// Sort sorts the collection.
+func (p ErrorList) Sort() {
+	sort.Sort(p)
+}
+
+func (p ErrorList) Error() string {
+	switch len(p) {
+	case 0:
+		return "no errors"
+	case 1:
+		return p[0].Error()
+	}
+	return fmt.Sprintf("%s (and %d more errors)", p[0], len(p)-1)
+}
+
+// Err returns an error.
+func (p ErrorList) Err() error {
+	if len(p) == 0 {
+		return nil
+	}
+
+	return p
+}

vendor/github.com/d5/tengo/compiler/parser/parse_file.go

@@ -0,0 +1,28 @@
+package parser
+
+import (
+	"io"
+
+	"github.com/d5/tengo/compiler/ast"
+	"github.com/d5/tengo/compiler/source"
+)
+
+// ParseFile parses a file with a given src.
+func ParseFile(file *source.File, src []byte, trace io.Writer) (res *ast.File, err error) {
+	p := NewParser(file, src, trace)
+
+	defer func() {
+		if e := recover(); e != nil {
+			if _, ok := e.(bailout); !ok {
+				panic(e)
+			}
+		}
+
+		p.errors.Sort()
+		err = p.errors.Err()
+	}()
+
+	res, err = p.ParseFile()
+
+	return
+}

vendor/github.com/d5/tengo/compiler/parser/parse_source.go

@@ -0,0 +1,16 @@
+package parser
+
+import (
+	"io"
+
+	"github.com/d5/tengo/compiler/ast"
+	"github.com/d5/tengo/compiler/source"
+)
+
+// ParseSource parses source code 'src' and builds an AST.
+func ParseSource(filename string, src []byte, trace io.Writer) (res *ast.File, err error) {
+	fileSet := source.NewFileSet()
+	file := fileSet.AddFile(filename, -1, len(src))
+
+	return ParseFile(file, src, trace)
+}

vendor/github.com/d5/tengo/compiler/parser/sync.go

@@ -0,0 +1,12 @@
+package parser
+
+import "github.com/d5/tengo/compiler/token"
+
+var stmtStart = map[token.Token]bool{
+	token.Break:    true,
+	token.Continue: true,
+	token.For:      true,
+	token.If:       true,
+	token.Return:   true,
+	token.Export:   true,
+}