PR for sourcemap support

Fixes #1 

Note: I've added sourcemap support for buffer using the instructions found [here](https://github.com/floridoo/gulp-sourcemaps#plugin-developers-only-how-to-add-source-map-support-to-plugins). For stream I couldn't find a way to make sourcemaps work, none of the other plugins that support `gulp-sourcemaps` allow streaming. Maybe this will change in the future but for now this should suffice.