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/script/compiled.go

@@ -0,0 +1,113 @@
+package script
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/d5/tengo/compiler"
+	"github.com/d5/tengo/objects"
+	"github.com/d5/tengo/runtime"
+)
+
+// Compiled is a compiled instance of the user script.
+// Use Script.Compile() to create Compiled object.
+type Compiled struct {
+	symbolTable *compiler.SymbolTable
+	machine     *runtime.VM
+}
+
+// Run executes the compiled script in the virtual machine.
+func (c *Compiled) Run() error {
+	return c.machine.Run()
+}
+
+// RunContext is like Run but includes a context.
+func (c *Compiled) RunContext(ctx context.Context) (err error) {
+	ch := make(chan error, 1)
+
+	go func() {
+		ch <- c.machine.Run()
+	}()
+
+	select {
+	case <-ctx.Done():
+		c.machine.Abort()
+		<-ch
+		err = ctx.Err()
+	case err = <-ch:
+	}
+
+	return
+}
+
+// IsDefined returns true if the variable name is defined (has value) before or after the execution.
+func (c *Compiled) IsDefined(name string) bool {
+	symbol, _, ok := c.symbolTable.Resolve(name)
+	if !ok {
+		return false
+	}
+
+	v := c.machine.Globals()[symbol.Index]
+	if v == nil {
+		return false
+	}
+
+	return *v != objects.UndefinedValue
+}
+
+// Get returns a variable identified by the name.
+func (c *Compiled) Get(name string) *Variable {
+	value := &objects.UndefinedValue
+
+	symbol, _, ok := c.symbolTable.Resolve(name)
+	if ok && symbol.Scope == compiler.ScopeGlobal {
+		value = c.machine.Globals()[symbol.Index]
+		if value == nil {
+			value = &objects.UndefinedValue
+		}
+	}
+
+	return &Variable{
+		name:  name,
+		value: value,
+	}
+}
+
+// GetAll returns all the variables that are defined by the compiled script.
+func (c *Compiled) GetAll() []*Variable {
+	var vars []*Variable
+	for _, name := range c.symbolTable.Names() {
+		symbol, _, ok := c.symbolTable.Resolve(name)
+		if ok && symbol.Scope == compiler.ScopeGlobal {
+			value := c.machine.Globals()[symbol.Index]
+			if value == nil {
+				value = &objects.UndefinedValue
+			}
+
+			vars = append(vars, &Variable{
+				name:  name,
+				value: value,
+			})
+		}
+	}
+
+	return vars
+}
+
+// Set replaces the value of a global variable identified by the name.
+// An error will be returned if the name was not defined during compilation.
+func (c *Compiled) Set(name string, value interface{}) error {
+	obj, err := objects.FromInterface(value)
+	if err != nil {
+		return err
+	}
+
+	symbol, _, ok := c.symbolTable.Resolve(name)
+	if !ok || symbol.Scope != compiler.ScopeGlobal {
+		return fmt.Errorf("'%s' is not defined", name)
+	}
+
+	c.machine.Globals()[symbol.Index] = &obj
+
+	return nil
+}