Rename NullStyleGuide -> UnknownStyleGuide

The lets us get rid of the `begin..rescue..end` clause.
Add initial implementation of language toggling

This is a first stab at implementing language toggling, mainly for the purpose
of letting users disable CoffeeScript linting. It makes RuboCop thow lots of
warnings about unused cops, and the method `language_enabled?` method might not
be the prettiest, but it seems to work.