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/closure.go

@@ -0,0 +1,45 @@
+package objects
+
+import (
+	"github.com/d5/tengo/compiler/token"
+)
+
+// Closure represents a function closure.
+type Closure struct {
+	Fn   *CompiledFunction
+	Free []*Object
+}
+
+// TypeName returns the name of the type.
+func (o *Closure) TypeName() string {
+	return "closure"
+}
+
+func (o *Closure) String() string {
+	return "<closure>"
+}
+
+// BinaryOp returns another object that is the result of
+// a given binary operator and a right-hand side object.
+func (o *Closure) BinaryOp(op token.Token, rhs Object) (Object, error) {
+	return nil, ErrInvalidOperator
+}
+
+// Copy returns a copy of the type.
+func (o *Closure) Copy() Object {
+	return &Closure{
+		Fn:   o.Fn.Copy().(*CompiledFunction),
+		Free: append([]*Object{}, o.Free...), // DO NOT Copy() of elements; these are variable pointers
+	}
+}
+
+// IsFalsy returns true if the value of the type is falsy.
+func (o *Closure) IsFalsy() bool {
+	return false
+}
+
+// Equals returns true if the value of the type
+// is equal to the value of another object.
+func (o *Closure) Equals(x Object) bool {
+	return false
+}