Inline source maps as comments

* rework(src, options) now accepts options arguments which is passed to
  css-parse, we set options.position = true so we can generate sourcemaps later

* Rework.prototype.toString(options) accepts sourcemap and sourcemapAsObject
  boolean options. The first one instructs css-stringify to generate sourcemap
  and then inlines it in source as base64 encoded data URI. If sourcemapAsObject
  is true then result from css-stringify returned unmodified and thus no
  sourcemap is inlined.