Add revision to XML extensions

This extension adds the revision field to the
XML tag for extensions. This allows a version
macro to be generated with:

* KhronosGroup/OpenCL-Headers#251
* KhronosGroup/OpenCL-Headers#248

Marked as **Draft** as I have a couple of open questions:
* Should we be adding this to vendor & ext extension entries in the
  XML too? Some vendor extensions aren't semantically versioned and
  are instead a value that's incremented, we could use this as the
  major value in semantic versioning.
* Should this revision entry be used to generate any part of the
  human readable specification? This would keep the values in sync
  rather than the specification revision being bumped and then the
  XML revision being forgotten about.

Author: Ewan Crawford