Add support for checking CoffeeScript

Adds support for checking CoffeeScript source files for style violations using
CoffeeLint.
Gracefully ignore files of an unknown language

Files of all languages will be passed to an instance of `StyleChecker`, and it
shouldn't break because it doesn't know how to check it for style violations.
Style fixes
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 throw lots of
warnings about unused cops, and the method `language_enabled?` method might not
be the prettiest, but it seems to work.
Merge remote-tracking branch 'koronen/vk-coffeescript'

Conflicts:
	Gemfile
	Gemfile.lock
	spec/models/style_checker_spec.rb
ignore redis junk
allow custom config for coffee
content -> contents typo
ignore method replacements
fix custom config loading
byebug -> pry
add rspec cops
use full cop names
use rspec cops
ignore broken exclude test
full rspec backtrace
force ssl