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
The boundaries between `StyleGuide::Base` and its subclasses are confusing to me.

* Children reference `StyleGuide::Base::CONFIG_DIR`
* Parent references children's `#uniq_messages_from_violations` and `#violations_per_line`
* Children reference parent's `repo_config`