[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, agreed. This plugin should _add_ `.min.js` and `.min.js.map` files to the input tree.

I can help you figure out how best to do this in Broccoli, but you'll effectively need to symlink/copy _everything_ over from the src dir to the dest dir. The thing I'll look up is whether or not it's advised to symlink dirs, or if we should always recurse into dirs and symlink/copy every file.