Re-structure style guide configuration

`.hound.yml` is now used to enable/disable language support and provide the
location of the configuration file.

Example `.hound.yml`:
```yaml
ruby:
  enabled: true
  config_file: .rubocop.yml

coffee_script:
  enabled: true
  config_file: config/coffeelint.json
```

* introduce `RepoConfig` class to handle configuration fetching and parsing
* extract `StyleGuide::Base` for common style guide logic
I don't necessarily thing it fits clearly in that paradigm (of grouping by language). The problem I see is that tests verify that with a certain condition one language returns true, but others return false. And it would be too wordy to split them out into separate cases. Unless we extract out the setup. Thoughts?