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
Unused method argument - `file`. If it's necessary, use `_` or `_file` as an argument name to indicate that it won't be used. You can also write as `excluded_file?(*)` if you want the method to accept any arguments but don't care about them.