Exclude dirs

This will allow you to specify directories you want excluded. I tried https://github.com/phaikawl/fresh which also attempts to add this feature, however it never worked quite right for me.

You add directories you want excluded via the runner.conf :
```
exclude_dir: assets, bower_components, something_else
```
@ae0000 The import path is breaking after merging this patch.

Ref. http://blog.campoy.cat/2014/03/github-and-go-forking-pull-requests-and.html