thoughtbot specific Hound configuration

https://trello.com/c/A08LSOcn/

Load in a thoughtbot specific configuration for all linters so that when changes to the thoughtbot style guides occur, hound can lint against the most up to day style guide, without affecting other customers.

Notes:

* Not sure if this is the best way to handle checking if a project is thoughtbot or not and to load a configuration based on this.
* Not sure if this is the best way to handle testing these cases as well.
* I simply copied over the existing configurations for rubocop, JShint, and Coffeelint into the new `config/style_guides/thoughtbot` directory. There are no changes to any of these configurations. I figured it could be handled as a separate PR. For example: https://github.com/thoughtbot/hound/pull/491

Any suggestions / feedback welcome. Especially around the first 2 notes.
Ah, good idea. I've extracted a `DefaultConfigFile` class as suggested. I left the tests in each style guide class (ruby, javascript, coffeescript) for now that explicitly check if the correct configuration file was used for each guide. What are your thoughts on this? I can't decide if it's overkill or useful.