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"
```
Style: We generally avoid naming things after the object type: https://github.com/thoughtbot/guides/tree/master/style#naming

What do you think about renaming `hash` to `options`?