Added ROOT_DIR configuration constant

Specify the ROOT_DIR to make the files get generated somewhere else. An
example use is how I have my test setup right now, it's very simple:

I have a test/ server which was spawned by harp sitting in my repository
folder (you can see it's ignored in the gitignore), so I set the
ROOT_DIR to be test/ and then I get everything spawned into my site that
way. Kinda handy.