Adds native globbing support. Fixes #51.

Took the liberty of fixing this as its a more intuitive behaviour. Would rather not have to recommend that people use `glob.sync` directly when using `gulp-uncss`. :smile:

Adds `is-html` to make sure the string is not HTML, if it isn't the path gets run through `glob`. Whether its a pattern or not, it will be expanded. Note that in one of the tests (Error reporting - No stylesheet found) the file will no longer be returned in the array, although the test still passes. I'm guessing this is because `glob` will check whether the file exists here too. It therefore might be worth removing the `existSync` test in this method then too, but I wasn't 100% sure on that.