Full upgrade replacement to Stylus 0.46.3
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
Remove test.js (accidentally added)
Add package.json to plugin
Add basic test cases and framework
Fix bug that breaks the shorthand rule matching
Remove console.log
Add README
Remove npm-debug.log
Change MediaShorthand from Stylus plugin to custom module

- The previous method of using Stylus plugin doesn't work with @imports.
- Added a test case to catch this as well.
- Updated the README
Remove npm-debug.log. Derp
Type check input
Merge pull request #1 from quizlet/upgrade-stylus

Upgrade stylus to 0.46.3