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/compiler_scopes.go

@@ -0,0 +1,43 @@
+package compiler
+
+import "github.com/d5/tengo/compiler/source"
+
+func (c *Compiler) currentInstructions() []byte {
+	return c.scopes[c.scopeIndex].instructions
+}
+
+func (c *Compiler) currentSourceMap() map[int]source.Pos {
+	return c.scopes[c.scopeIndex].sourceMap
+}
+
+func (c *Compiler) enterScope() {
+	scope := CompilationScope{
+		symbolInit: make(map[string]bool),
+		sourceMap:  make(map[int]source.Pos),
+	}
+
+	c.scopes = append(c.scopes, scope)
+	c.scopeIndex++
+
+	c.symbolTable = c.symbolTable.Fork(false)
+
+	if c.trace != nil {
+		c.printTrace("SCOPE", c.scopeIndex)
+	}
+}
+
+func (c *Compiler) leaveScope() (instructions []byte, sourceMap map[int]source.Pos) {
+	instructions = c.currentInstructions()
+	sourceMap = c.currentSourceMap()
+
+	c.scopes = c.scopes[:len(c.scopes)-1]
+	c.scopeIndex--
+
+	c.symbolTable = c.symbolTable.Parent(true)
+
+	if c.trace != nil {
+		c.printTrace("SCOPL", c.scopeIndex)
+	}
+
+	return
+}