Dedicated syntax highlighting server for cgit.

Use a simple HTTP server to highlight source for cgit. Improves performance
compared to invoking python on every request.

Usage:

1. Run `make install`.
2. Configure your system to run /usr/lib/cgit/syntax-highlighting-server.py at
   boot. This can be done by `systemctl enable syntax-highlighting` on systemd
   machines.
3. Set your cgit source filter to syntax-highlighting-client.sh.