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
How about just defining `name` (or a constant) on the child classes? If the logic for `enabled?` ever changed, it would cause "shotgun surgery" style changes in multiple files.