Refactored prefixing
Prefixing requirements now refer to browsers, which refer to their prefix
Support can be turned off for specific browser groups
Automatic prefixing can now be completely turned off
The prefix mixin now takes a list of browsers to NOT prefix
Moved flexbox mixins to core/prefixes/
Renamed core/prefixes/ to core/support/
Reflects how the support module is no longer just about prefixing
New mixins for flexbox
Each flexbox property has its own mixin
Logic wrapped inside each mixin
flex and flex-item now just call all their mixins