Add "exclude" option

[antoi]

"exclude" option is directory or files to ignore.

Use exclude to ignore libraries in dev path.

[mazerte]

Thanks for your Pull Request !

But it's possible merge your two PR in one.
And it's better if you add some unit test for validate your code. Add a new sub-task in the Grunt file and some fixtures.

Please note @Philzen work on it ( #3 ), I validate the PR for the first who add some tests 😄

Thanks again.

[mazerte]

@Philzen push some tests, it's good for you ?