feat(sourcemaps): provide option for writing out external sourcemaps

This PR introduces a new `sourceMapDest` configuration option for gulp-sass. If a sourcemap has been produced, and this option is specified, gulp-sass will synchronously write the sourcemap contents to a file stream and updating the CSS content stream with a path-less reference (since gulp stream destination is ultimately unknown).

This might not be ideal for everyone, but it unlocks functionality that is useful in a variety of situations.