Add ability to specify subdirectory to run leiningen in

Add the ability to specify a subdirectory to run the leiningen task in. By default this option is hidden under an 'advanced' section.

![screen shot 2013-09-03 at 10 29 52](https://f.cloud.github.com/assets/301092/1071297/118952ce-1473-11e3-9994-605fff8bf5b4.png)