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/objects/bytes.go

@@ -0,0 +1,76 @@
+package objects
+
+import (
+	"bytes"
+
+	"github.com/d5/tengo/compiler/token"
+)
+
+// Bytes represents a byte array.
+type Bytes struct {
+	Value []byte
+}
+
+func (o *Bytes) String() string {
+	return string(o.Value)
+}
+
+// TypeName returns the name of the type.
+func (o *Bytes) TypeName() string {
+	return "bytes"
+}
+
+// BinaryOp returns another object that is the result of
+// a given binary operator and a right-hand side object.
+func (o *Bytes) BinaryOp(op token.Token, rhs Object) (Object, error) {
+	switch op {
+	case token.Add:
+		switch rhs := rhs.(type) {
+		case *Bytes:
+			return &Bytes{Value: append(o.Value, rhs.Value...)}, nil
+		}
+	}
+
+	return nil, ErrInvalidOperator
+}
+
+// Copy returns a copy of the type.
+func (o *Bytes) Copy() Object {
+	return &Bytes{Value: append([]byte{}, o.Value...)}
+}
+
+// IsFalsy returns true if the value of the type is falsy.
+func (o *Bytes) IsFalsy() bool {
+	return len(o.Value) == 0
+}
+
+// Equals returns true if the value of the type
+// is equal to the value of another object.
+func (o *Bytes) Equals(x Object) bool {
+	t, ok := x.(*Bytes)
+	if !ok {
+		return false
+	}
+
+	return bytes.Compare(o.Value, t.Value) == 0
+}
+
+// IndexGet returns an element (as Int) at a given index.
+func (o *Bytes) IndexGet(index Object) (res Object, err error) {
+	intIdx, ok := index.(*Int)
+	if !ok {
+		err = ErrInvalidIndexType
+		return
+	}
+
+	idxVal := int(intIdx.Value)
+
+	if idxVal < 0 || idxVal >= len(o.Value) {
+		res = UndefinedValue
+		return
+	}
+
+	res = &Int{Value: int64(o.Value[idxVal])}
+
+	return
+}