Strips the primary file extension before appending the source extension attached to the language, which is derived in a few different ways:

* If `.*.md` this will be `""`
* If `.litcoffee` this will be `".coffee"` (see [languages.json changes](https://github.com/jashkenas/docco/pull/199/files#r3752382))
* Otherwise, this will be the same as the current file