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/source/file_set.go

@@ -0,0 +1,96 @@
+package source
+
+import (
+	"sort"
+)
+
+// FileSet represents a set of source files.
+type FileSet struct {
+	Base     int     // base offset for the next file
+	Files    []*File // list of files in the order added to the set
+	LastFile *File   // cache of last file looked up
+}
+
+// NewFileSet creates a new file set.
+func NewFileSet() *FileSet {
+	return &FileSet{
+		Base: 1, // 0 == NoPos
+	}
+}
+
+// AddFile adds a new file in the file set.
+func (s *FileSet) AddFile(filename string, base, size int) *File {
+	if base < 0 {
+		base = s.Base
+	}
+	if base < s.Base || size < 0 {
+		panic("illegal base or size")
+	}
+
+	f := &File{
+		set:   s,
+		Name:  filename,
+		Base:  base,
+		Size:  size,
+		Lines: []int{0},
+	}
+
+	base += size + 1 // +1 because EOF also has a position
+	if base < 0 {
+		panic("offset overflow (> 2G of source code in file set)")
+	}
+
+	// add the file to the file set
+	s.Base = base
+	s.Files = append(s.Files, f)
+	s.LastFile = f
+
+	return f
+}
+
+// File returns the file that contains the position p.
+// If no such file is found (for instance for p == NoPos),
+// the result is nil.
+//
+func (s *FileSet) File(p Pos) (f *File) {
+	if p != NoPos {
+		f = s.file(p)
+	}
+
+	return
+}
+
+// Position converts a Pos p in the fileset into a FilePos value.
+func (s *FileSet) Position(p Pos) (pos FilePos) {
+	if p != NoPos {
+		if f := s.file(p); f != nil {
+			return f.position(p)
+		}
+	}
+
+	return
+}
+
+func (s *FileSet) file(p Pos) *File {
+	// common case: p is in last file
+	if f := s.LastFile; f != nil && f.Base <= int(p) && int(p) <= f.Base+f.Size {
+		return f
+	}
+
+	// p is not in last file - search all files
+	if i := searchFiles(s.Files, int(p)); i >= 0 {
+		f := s.Files[i]
+
+		// f.base <= int(p) by definition of searchFiles
+		if int(p) <= f.Base+f.Size {
+			s.LastFile = f // race is ok - s.last is only a cache
+			return f
+		}
+	}
+
+	return nil
+}
+
+func searchFiles(a []*File, x int) int {
+	return sort.Search(len(a), func(i int) bool { return a[i].Base > x }) - 1
+}