Adding support for automatic detection of retina images.

- Automatic detection of {name}.@2x.{ext} retina images in url();
- Available via a retina option

Original author @asuth. See:
https://github.com/quizlet/stylus/compare/53330c03fe88f5aaa8f67c832fa89598654b0c1b...a127c867a5429b9ae94bb85000d85ab188b91f76
Add plugin hook before parser
Add mediashorthand plugin