Basic sourcemaps support

You should provide a npm API to recover the sourcemap without persisting it somewhere on the filesystem, so that third-party build systems could use it. See for instance how the LESS sourcemap is used by the Meteor build system: https://github.com/meteor/meteor/blob/3d75f53149183d0ec60ccdf33faf75fcb8d6729c/packages/less/plugin/compile-less.js#L60-L64