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/ast/file.go

@@ -0,0 +1,32 @@
+package ast
+
+import (
+	"strings"
+
+	"github.com/d5/tengo/compiler/source"
+)
+
+// File represents a file unit.
+type File struct {
+	InputFile *source.File
+	Stmts     []Stmt
+}
+
+// Pos returns the position of first character belonging to the node.
+func (n *File) Pos() source.Pos {
+	return source.Pos(n.InputFile.Base)
+}
+
+// End returns the position of first character immediately after the node.
+func (n *File) End() source.Pos {
+	return source.Pos(n.InputFile.Base + n.InputFile.Size)
+}
+
+func (n *File) String() string {
+	var stmts []string
+	for _, e := range n.Stmts {
+		stmts = append(stmts, e.String())
+	}
+
+	return strings.Join(stmts, "; ")
+}