add skipCommit option

[mikrofusion]

First off, very cool gulp plugin.

There are projects where nothing is 'compiled' as part of the process of publishing the code. In these cases, because there are no changes gulp-subtree tries to do a commit (which fails) and then resets HEAD~1 (which unstages the last commit). To get around this I'd like to propose a skipCommit option that can be used for projects that require no additional compilation.

Please let me know what you think.

• mike