Don't crash if config={}

The signature of `docco.parse` is `(source, code, config={})` but `getLanguage` crashes if config is {}.  Fix this.

This is a regression which appeared in bb4df7546b248adc47db89d5660d5533148b5fc1 (docco 0.6.3).  In docco 0.6.2 the signature of `docco.parse` was `(source, code)` and it didn't crash.
Merge pull request #291 from cscott/patch-1

docco.parse: Don't crash if config parameter is default value.