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
`hound_config` now sorta means config from `.hound.yml`. And we really never used `pull_request_config` when talking about the code, but we did use the word "custom". I thought this would make a bit more sense with these connotations in mind.