Add detection for Windows 8.1

This adds a new token for Windows 8.1 (`win8_1`) as well as a new token for Windows 8.0 (`win8_0`) so that they can be detected independently but the general token (`win8`) can be used for both.

There were a bunch of differences in the platform that would be useful to detect between for, say, links advertising Windows 8 applications (say the application only worked in Windows 8.1).