Support including other config files

This adds support for including a RuboCop config file inside of `.hound.yml`.
Last seen Cop configuration will overwrite any previous ones.

Example `.hound.yml`:

```yaml
Include:
  - config/.rubocop.yml
  - .rubocop.yml

Style/LineLength:
  Max: 100
```
The fact that `file_content` returns `test: blah`, but this context is unnecessary, since there is no other context.