Adds XML5 parser based on [spec](https://github.com/annevk/xml5) by
. Currently working draft resides on
https://github.com/Ygg01/xml5_draft and is rendered using
[Bikeshed](https://github.com/tabatkins/bikeshed).

This patch is only concerned with making it work right, according to
spec. Things to be done:

1. Add some support for doctype as suggested by annevk
    here:  Ygg01/xml5_draft#2
2. Finish references in xml5 - basically, use all entity
   replacements html5 uses and add test for those.
3. Add Namespace support.
4. Unify two parsers using associated types.
5. Add C API for xml5 parser.