Replace fetch with square brackets element reference
Construct *StyleGuide objects using meta-programming

Adds a method `ModifiedFile#language`, which will be replaced with the
implementation in #275 when merged.
Update names of custom config files

Make `.hound` a common prefix and add the language name before the file
extension. This is a step towards making the names of the config files stable
while the underlying tool might change.
Add info about custom CoffeeScript config to view