Add Movable Type Markup Language support

[Movable Type](http://movabletype.org/), the content management system started more than a decade ago, commonly uses the `.mtml` extension to represent files written in its template language, based on XML and commonly intertwined with HTML.

There are [almost 10K](https://github.com/search?q=mt+extension%3Amtml&type=Code&ref=searchresults) such files on Github.