[experimental] Add support for sourceMap

* removes filter since that will not help with the 1 to 2 or 2 to 2 mapping
* adds `sourceMap` option to force the generation of `.map` file per `.js` file in tree
* produces `.map` file automatically if there is an existing `.map` file from previous transformations in the tree

Note: This PR depends on https://github.com/mishoo/UglifyJS2/pull/580

TODO: 

* [ ] cache (if needed)
* [ ] revisit the default value of `sourceMap` option.

/cc @ericf @eventualbuddha

Should this be configurable so people can output to `.min.js`?