Sourcemap support

[zenflow]

Work (somehow) with gulp-sourcemaps to produce sourcemaps.

This probably won't be so cut-and-dry as with other transforms since the content type of the source (css) is different from the output (js). Still I think there is probably a way.

[fidian]

This sounds like a good idea. I won't be implementing it, but I would gladly accept pull requests for this functionality.

I've moved on from grunt/gulp to metalsmith and have simultaneously decreased my build times and simplified my build process. This plugin will still be maintained even though I don't use it any longer. However, maintenance is limited to accepting pull requests and addressing bugs.