Intial version of box-decoration-break JSON

A few notes:

1. Contrary to info on the web, IE does not seem to support this feature.
2. I wasn’t sure how far back support in WebKit-based browsers goes, so I took the year 2013 as a starting point for `"y x"` in Chrome/Safari/Android.

Related Autoprefixer issue: https://github.com/postcss/autoprefixer/issues/344
Demo of `box-decoration-break`: http://jsbin.com/xojoro/edit?css,output