[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

yeah, same question here. but now that you have the question about .min.js, that means the whole tree should be copied over to maintain the original .js and the original .js.map, in which case I don't think we really need to copy everything, but just modify the current tree, but that I don't know how to do in broccoli.