Make source deploy folder configurable

Hi,

I recently used the grunt-deploy plugin and it works great. One thing I found is that I want to deploy only part of my project. Typically in a Grunt/bower project there is a build phase, e.g. 'dist' which has all the files needed for deploy.

With my code change, and an added lite to the Gruntfile.js, the deploys are a bit faster;
source_path: 'dist/.',

I did not add any tests, but the change is small and it seems to work as intended. So I'd be happy if you pulled it.

Thanks for a great plugin,
Martin