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/objects/errors.go

@@ -0,0 +1,32 @@
+package objects
+
+import (
+	"errors"
+	"fmt"
+)
+
+// ErrIndexOutOfBounds is an error where a given index is out of the bounds.
+var ErrIndexOutOfBounds = errors.New("index out of bounds")
+
+// ErrInvalidIndexType represents an invalid index type.
+var ErrInvalidIndexType = errors.New("invalid index type")
+
+// ErrInvalidIndexValueType represents an invalid index value type.
+var ErrInvalidIndexValueType = errors.New("invalid index value type")
+
+// ErrInvalidOperator represents an error for invalid operator usage.
+var ErrInvalidOperator = errors.New("invalid operator")
+
+// ErrWrongNumArguments represents a wrong number of arguments error.
+var ErrWrongNumArguments = errors.New("wrong number of arguments")
+
+// ErrInvalidArgumentType represents an invalid argument value type error.
+type ErrInvalidArgumentType struct {
+	Name     string
+	Expected string
+	Found    string
+}
+
+func (e ErrInvalidArgumentType) Error() string {
+	return fmt.Sprintf("invalid type for argument '%s': expected %s, found %s", e.Name, e.Expected, e.Found)
+}