The faces should really use foreground colors.

Regression introduced in 8a2c0374b0.

Fixes #2.
Ignore blacklisted delimiters altogether.

Blacklisted delimiters don't contribute to depth anymore.
Properly handle cleaning up the 'category' property.

This comes with a redesign: instead of setting the 'category' property
in a 'syntax-propertize-function', it's done in our font-lock handler.
Removing them is done in a 'font-lock-unfontify-region-function'.