Add support for sourceMap

* removes filter since that will not help with the 1 to 2 or 2 to 2 mapping
* adds support for source map by generating of `.min.js.map` file per `.min.js` file generated into the tree
* composed source map if there is an existing `.js.map` file from previous transformations in the tree

Note: This PR depends on https://github.com/mishoo/UglifyJS2/pull/580

TODO: 

* [x] cache (if needed)

/cc @ericf @eventualbuddha

creating a cache entry before starting the uglify process, it might sounds weird, but should not be a problem... :), we only store metadata in the cache anyway.