Allow to map a filename to a language based on a prefix

[jrieken]

In addition to the extname (*.es6), the full name (Dockerfile), and the first line content (#! /user/bin/node) we should also allow to map a file to a language based on a prefix, like Dockerfile-dev maps because of Dockerfile*.

[jrieken]

@PeterJausovec, @SteveLasker fyi

[matthewp]

What do you mean by prefix? I'd like to be able to map file extensions to a language, ie map .foo to HTML.

[egamma]

@matthewp this is another issue #334, let's keep these issues separate.