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

The call to `hashStrings()` is not needed, because `hashTree()` uses the file's relative part as part of the hash's input. https://github.com/broccolijs/broccoli-kitchen-sink-helpers/blob/master/index.js#L72-75