Allow a user to exclude certain JavaScript files

Allows a user to configure hound to ignore specified JavaScript files
from being run through the jshint through an "exclude" configuration.

Example configuration:

```yaml
java_script:
  enabled: true
  exclude:
    - "app/assets/javascripts/*.js"
```
Thoughts on naming this function more general, like `javascript_ignored_files`? The reason being is that we're not married to JSHint, and may switch to another/better linter any time.