Log Message:
gumbo-parser: Update to 0.12.1 and change to continuation fork

Update prepared in wip by Kevin Bloom.

0.12.0 -> 0.12.1:
- Fix empty the end of the tag <form>
- Correctly handle text in <form> elements
- Fix tag name filtering that could result in XSS
- Return visualc/include/strings.h file

0.11.1 -> 0.12.0:
- Add support for <dialog> tag
- Fix TAGSET_INCLUDES macro to work properly with multiple bit flags

0.10.1 -> 0.11.0:
- Add support for <picture> tag
- Make genperf.py script compatible with Python 3
- Change maintainer to Grigory Kirillov