2.3.0

Changes:
- Overhaul of selectors API
- New experimental rule for hidden content
- New rule for aria-hidden="true" on the body
- Color-contrast rule impact is now serious
- Color-contrast fixes for implicit labels and TR elements
- Color-contrast puts 1:1 ratio elements into Needs Review/incomplete
- List category mappings in docs
- Update axe.source to work with Firefox webdriver