Added ability to split source maps into separate files.

This makes it easier to feed these source maps into other parts of builds like uglify that, as of present, do not support reading inline source maps.

This is kind of half-baked. The tests pass but I didn't add any for the additional functionality. I'm also working to make sure this works I intend it to as well, but I thought at worst I'd share an idea with you. :smile: :bulb: