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/map_iterator.go

@@ -0,0 +1,62 @@
+package objects
+
+import "github.com/d5/tengo/compiler/token"
+
+// MapIterator represents an iterator for the map.
+type MapIterator struct {
+	v map[string]Object
+	k []string
+	i int
+	l int
+}
+
+// TypeName returns the name of the type.
+func (i *MapIterator) TypeName() string {
+	return "map-iterator"
+}
+
+func (i *MapIterator) String() string {
+	return "<map-iterator>"
+}
+
+// BinaryOp returns another object that is the result of
+// a given binary operator and a right-hand side object.
+func (i *MapIterator) BinaryOp(op token.Token, rhs Object) (Object, error) {
+	return nil, ErrInvalidOperator
+}
+
+// IsFalsy returns true if the value of the type is falsy.
+func (i *MapIterator) IsFalsy() bool {
+	return true
+}
+
+// Equals returns true if the value of the type
+// is equal to the value of another object.
+func (i *MapIterator) Equals(Object) bool {
+	return false
+}
+
+// Copy returns a copy of the type.
+func (i *MapIterator) Copy() Object {
+	return &MapIterator{v: i.v, k: i.k, i: i.i, l: i.l}
+}
+
+// Next returns true if there are more elements to iterate.
+func (i *MapIterator) Next() bool {
+	i.i++
+	return i.i <= i.l
+}
+
+// Key returns the key or index value of the current element.
+func (i *MapIterator) Key() Object {
+	k := i.k[i.i-1]
+
+	return &String{Value: k}
+}
+
+// Value returns the value of the current element.
+func (i *MapIterator) Value() Object {
+	k := i.k[i.i-1]
+
+	return i.v[k]
+}