Adds plugin to clean attrs added by Sketch

I created a plugin to remove all the attributes added by [Sketch](http://www.bohemiancoding.com/sketch/) a very popular vector design application for Mac. I'm not really sure that this is the best way to implement this but it works and tests are passing.

Is this something you might want to have out of the box? If not what is the best way to distribute and use plugins from the community?