Re-enable sourcemaps to work together with cascade option including tests and docs

`options` object needs to be passed to Autoprefixer.process() as well for some options (like maps) to work.
See [autoprefixer-core/blob/2.2.0/lib/autoprefixer.coffee#L47](https://github.com/postcss/autoprefixer-core/blob/2.2.0/lib/autoprefixer.coffee#L47)