Adds support for .pyxy files, which allow writing HTML in Python.
See https://github.com/pyxy-org/pyxy
- Fix bug on hyphenated tag names
- Partially fixed, need to figure out how to handle tag names which aren't valid identifiers (due to containing a hyphen, colon, or otherwise)
- Validate closing tag name
- Highlight matching tag pairs
- Colorize nested tags (like is done for HTML files)