add chat/catgirl

catgirl(1) is a TLS-only terminal IRC client.

Notable Features:

* Tab complete: most recently seen or mentioned nicks are completed first.
Commas are inserted between multiple nicks.

* Prompt: the prompt clearly shows whether input will be interpreted as a
command or sent as a message.

* Split scroll: keeps the latest messages in view while scrolling.

* URL detection: recent URLs from a particular user or matching a substring
can be opened or copied.

* Nick coloring: color generation based on usernames remains stable across
nick changes. Mentions of users in messages are colored.

* Topic diffing: the modified portion of a channel topic change is highlighted.

* Ignore: visibility of filtered messages can be toggled.

(nia)