Generate rem fallbacks only if needed

This won't generate fallbacks anymore when browsers without `rem` support won't understand the property/value, as explained in #5 (eg in media-queries (all at-rules actually), in properties like `transform`, `border-radius`, etc. and in values like `calc()` or `linear-gradient()`)

To keep this module simple and fast, I choose to not use caniuse data. I rather prefer to add cases manually. Reading data from caniuse to get all browsers that do not understand `rem` is easy, but determine for each browser if a particular feature is supported can be tricky, and in many cases not so obvious.

I also added an option (`atrules`) to keep fallbacks in atrules (if tools like respond.js are used for example)
Updated to PostCSS ^3.0.0
Get root font-size value if defined in CSS
Remove deprecation note
Deal with prefixes and make pixrem order-independent of Autoprefixer