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"
```
I believe the `strip_heredoc` will take care of stripping the indenting. The indenting I was proposing was purely for style issues (i.e. we indent things inside of blocks).