Write source map into its own file

If `toString.sourcemapAsObject` is set to `true` the resulting CSS was garbage before, as it only contained `[object Object]`. 

Now it writes the source map into its own file using some (optional) settings taken from https://github.com/gruntjs/grunt-contrib-less/blob/master/README.md#sourcemap.
These are: `sourceMapFilename`, `sourceMapURL` and `sourceMapRootpath`.

This could be seen as a _future polyfill_ as I hope this will be a core feature soon. See https://github.com/reworkcss/rework/issues/176.