This is semi-related to our discussions recently regarding methods operating on files/maps/both, but it strikes me as a bit odd that this method returns the `url` to append to the file *and* writes the source map. It seems to me that it might be doing too much. How do we handle the situation where it can't write the map for some reason (permission, etc.)? Would it still return the url?

The alternative is having more code in `tasks/concat.js` to make two separate calls, but it might make more sense in this situation.

Thoughts @mzgoddard?