Sourcemap support

This is a first pass at adding sourcemap support, as requested in #49.  There is no sourcemap combination functionality, so if the `--sourcemap` option is used then it will erase any existing sourcemaps from previous steps.  This can possibly be added later in a fashion similar to e.g. `ai/autoprefixer`.

Otherwise, it should work fine.  Please try it out and check the results at http://sokra.github.io/source-map-visualization/ -- I tested with the `tests/original.js` test file and all of the output is as expected.