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.go

@@ -0,0 +1,110 @@
+package source
+
+// File represents a source file.
+type File struct {
+	// File set for the file
+	set *FileSet
+	// File name as provided to AddFile
+	Name string
+	// Pos value range for this file is [base...base+size]
+	Base int
+	// File size as provided to AddFile
+	Size int
+	// Lines contains the offset of the first character for each line (the first entry is always 0)
+	Lines []int
+}
+
+// Set returns FileSet.
+func (f *File) Set() *FileSet {
+	return f.set
+}
+
+// LineCount returns the current number of lines.
+func (f *File) LineCount() int {
+	return len(f.Lines)
+}
+
+// AddLine adds a new line.
+func (f *File) AddLine(offset int) {
+	if i := len(f.Lines); (i == 0 || f.Lines[i-1] < offset) && offset < f.Size {
+		f.Lines = append(f.Lines, offset)
+	}
+}
+
+// LineStart returns the position of the first character in the line.
+func (f *File) LineStart(line int) Pos {
+	if line < 1 {
+		panic("illegal line number (line numbering starts at 1)")
+	}
+
+	if line > len(f.Lines) {
+		panic("illegal line number")
+	}
+
+	return Pos(f.Base + f.Lines[line-1])
+}
+
+// FileSetPos returns the position in the file set.
+func (f *File) FileSetPos(offset int) Pos {
+	if offset > f.Size {
+		panic("illegal file offset")
+	}
+
+	return Pos(f.Base + offset)
+}
+
+// Offset translates the file set position into the file offset.
+func (f *File) Offset(p Pos) int {
+	if int(p) < f.Base || int(p) > f.Base+f.Size {
+		panic("illegal Pos value")
+	}
+
+	return int(p) - f.Base
+}
+
+// Position translates the file set position into the file position.
+func (f *File) Position(p Pos) (pos FilePos) {
+	if p != NoPos {
+		if int(p) < f.Base || int(p) > f.Base+f.Size {
+			panic("illegal Pos value")
+		}
+
+		pos = f.position(p)
+	}
+
+	return
+}
+
+func (f *File) position(p Pos) (pos FilePos) {
+	offset := int(p) - f.Base
+	pos.Offset = offset
+	pos.Filename, pos.Line, pos.Column = f.unpack(offset)
+
+	return
+}
+
+func (f *File) unpack(offset int) (filename string, line, column int) {
+	filename = f.Name
+	if i := searchInts(f.Lines, offset); i >= 0 {
+		line, column = i+1, offset-f.Lines[i]+1
+	}
+
+	return
+}
+
+func searchInts(a []int, x int) int {
+	// This function body is a manually inlined version of:
+	//   return sort.Search(len(a), func(i int) bool { return a[i] > x }) - 1
+	i, j := 0, len(a)
+	for i < j {
+		h := i + (j-i)/2 // avoid overflow when computing h
+		// i ≤ h < j
+		if a[h] <= x {
+			i = h + 1
+		} else {
+			j = h
+		}
+	}
+
+	return i - 1
+}