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/compiler_assign.go

@@ -0,0 +1,133 @@
+package compiler
+
+import (
+	"fmt"
+
+	"github.com/d5/tengo/compiler/ast"
+	"github.com/d5/tengo/compiler/token"
+)
+
+func (c *Compiler) compileAssign(node ast.Node, lhs, rhs []ast.Expr, op token.Token) error {
+	numLHS, numRHS := len(lhs), len(rhs)
+	if numLHS > 1 || numRHS > 1 {
+		return c.errorf(node, "tuple assignment not allowed")
+	}
+
+	// resolve and compile left-hand side
+	ident, selectors := resolveAssignLHS(lhs[0])
+	numSel := len(selectors)
+
+	if op == token.Define && numSel > 0 {
+		// using selector on new variable does not make sense
+		return c.errorf(node, "operator ':=' not allowed with selector")
+	}
+
+	symbol, depth, exists := c.symbolTable.Resolve(ident)
+	if op == token.Define {
+		if depth == 0 && exists {
+			return c.errorf(node, "'%s' redeclared in this block", ident)
+		}
+
+		symbol = c.symbolTable.Define(ident)
+	} else {
+		if !exists {
+			return c.errorf(node, "unresolved reference '%s'", ident)
+		}
+	}
+
+	// +=, -=, *=, /=
+	if op != token.Assign && op != token.Define {
+		if err := c.Compile(lhs[0]); err != nil {
+			return err
+		}
+	}
+
+	// compile RHSs
+	for _, expr := range rhs {
+		if err := c.Compile(expr); err != nil {
+			return err
+		}
+	}
+
+	switch op {
+	case token.AddAssign:
+		c.emit(node, OpAdd)
+	case token.SubAssign:
+		c.emit(node, OpSub)
+	case token.MulAssign:
+		c.emit(node, OpMul)
+	case token.QuoAssign:
+		c.emit(node, OpDiv)
+	case token.RemAssign:
+		c.emit(node, OpRem)
+	case token.AndAssign:
+		c.emit(node, OpBAnd)
+	case token.OrAssign:
+		c.emit(node, OpBOr)
+	case token.AndNotAssign:
+		c.emit(node, OpBAndNot)
+	case token.XorAssign:
+		c.emit(node, OpBXor)
+	case token.ShlAssign:
+		c.emit(node, OpBShiftLeft)
+	case token.ShrAssign:
+		c.emit(node, OpBShiftRight)
+	}
+
+	// compile selector expressions (right to left)
+	for i := numSel - 1; i >= 0; i-- {
+		if err := c.Compile(selectors[i]); err != nil {
+			return err
+		}
+	}
+
+	switch symbol.Scope {
+	case ScopeGlobal:
+		if numSel > 0 {
+			c.emit(node, OpSetSelGlobal, symbol.Index, numSel)
+		} else {
+			c.emit(node, OpSetGlobal, symbol.Index)
+		}
+	case ScopeLocal:
+		if numSel > 0 {
+			c.emit(node, OpSetSelLocal, symbol.Index, numSel)
+		} else {
+			if op == token.Define && !symbol.LocalAssigned {
+				c.emit(node, OpDefineLocal, symbol.Index)
+			} else {
+				c.emit(node, OpSetLocal, symbol.Index)
+			}
+		}
+
+		// mark the symbol as local-assigned
+		symbol.LocalAssigned = true
+	case ScopeFree:
+		if numSel > 0 {
+			c.emit(node, OpSetSelFree, symbol.Index, numSel)
+		} else {
+			c.emit(node, OpSetFree, symbol.Index)
+		}
+	default:
+		panic(fmt.Errorf("invalid assignment variable scope: %s", symbol.Scope))
+	}
+
+	return nil
+}
+
+func resolveAssignLHS(expr ast.Expr) (name string, selectors []ast.Expr) {
+	switch term := expr.(type) {
+	case *ast.SelectorExpr:
+		name, selectors = resolveAssignLHS(term.Expr)
+		selectors = append(selectors, term.Sel)
+		return
+
+	case *ast.IndexExpr:
+		name, selectors = resolveAssignLHS(term.Expr)
+		selectors = append(selectors, term.Index)
+
+	case *ast.Ident:
+		name = term.Name
+	}
+
+	return
+}