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/bytecode.go

@@ -0,0 +1,134 @@
+package compiler
+
+import (
+	"encoding/gob"
+	"fmt"
+	"io"
+	"reflect"
+
+	"github.com/d5/tengo/compiler/source"
+	"github.com/d5/tengo/objects"
+)
+
+// Bytecode is a compiled instructions and constants.
+type Bytecode struct {
+	FileSet      *source.FileSet
+	MainFunction *objects.CompiledFunction
+	Constants    []objects.Object
+}
+
+// Decode reads Bytecode data from the reader.
+func (b *Bytecode) Decode(r io.Reader) error {
+	dec := gob.NewDecoder(r)
+
+	if err := dec.Decode(&b.FileSet); err != nil {
+		return err
+	}
+	// TODO: files in b.FileSet.File does not have their 'set' field properly set to b.FileSet
+	// as it's private field and not serialized by gob encoder/decoder.
+
+	if err := dec.Decode(&b.MainFunction); err != nil {
+		return err
+	}
+
+	if err := dec.Decode(&b.Constants); err != nil {
+		return err
+	}
+
+	// replace Bool and Undefined with known value
+	for i, v := range b.Constants {
+		b.Constants[i] = cleanupObjects(v)
+	}
+
+	return nil
+}
+
+// Encode writes Bytecode data to the writer.
+func (b *Bytecode) Encode(w io.Writer) error {
+	enc := gob.NewEncoder(w)
+
+	if err := enc.Encode(b.FileSet); err != nil {
+		return err
+	}
+
+	if err := enc.Encode(b.MainFunction); err != nil {
+		return err
+	}
+
+	// constants
+	return enc.Encode(b.Constants)
+}
+
+// FormatInstructions returns human readable string representations of
+// compiled instructions.
+func (b *Bytecode) FormatInstructions() []string {
+	return FormatInstructions(b.MainFunction.Instructions, 0)
+}
+
+// FormatConstants returns human readable string representations of
+// compiled constants.
+func (b *Bytecode) FormatConstants() (output []string) {
+	for cidx, cn := range b.Constants {
+		switch cn := cn.(type) {
+		case *objects.CompiledFunction:
+			output = append(output, fmt.Sprintf("[% 3d] (Compiled Function|%p)", cidx, &cn))
+			for _, l := range FormatInstructions(cn.Instructions, 0) {
+				output = append(output, fmt.Sprintf("     %s", l))
+			}
+		default:
+			output = append(output, fmt.Sprintf("[% 3d] %s (%s|%p)", cidx, cn, reflect.TypeOf(cn).Elem().Name(), &cn))
+		}
+	}
+
+	return
+}
+
+func cleanupObjects(o objects.Object) objects.Object {
+	switch o := o.(type) {
+	case *objects.Bool:
+		if o.IsFalsy() {
+			return objects.FalseValue
+		}
+		return objects.TrueValue
+	case *objects.Undefined:
+		return objects.UndefinedValue
+	case *objects.Array:
+		for i, v := range o.Value {
+			o.Value[i] = cleanupObjects(v)
+		}
+	case *objects.Map:
+		for k, v := range o.Value {
+			o.Value[k] = cleanupObjects(v)
+		}
+	}
+
+	return o
+}
+
+func init() {
+	gob.Register(&source.FileSet{})
+	gob.Register(&source.File{})
+	gob.Register(&objects.Array{})
+	gob.Register(&objects.ArrayIterator{})
+	gob.Register(&objects.Bool{})
+	gob.Register(&objects.Break{})
+	gob.Register(&objects.BuiltinFunction{})
+	gob.Register(&objects.Bytes{})
+	gob.Register(&objects.Char{})
+	gob.Register(&objects.Closure{})
+	gob.Register(&objects.CompiledFunction{})
+	gob.Register(&objects.Continue{})
+	gob.Register(&objects.Error{})
+	gob.Register(&objects.Float{})
+	gob.Register(&objects.ImmutableArray{})
+	gob.Register(&objects.ImmutableMap{})
+	gob.Register(&objects.Int{})
+	gob.Register(&objects.Map{})
+	gob.Register(&objects.MapIterator{})
+	gob.Register(&objects.ReturnValue{})
+	gob.Register(&objects.String{})
+	gob.Register(&objects.StringIterator{})
+	gob.Register(&objects.Time{})
+	gob.Register(&objects.Undefined{})
+	gob.Register(&objects.UserFunction{})
+}