circe-ctcp-CLIENTINFO-handler: support this CTCP request

This replies with a list of supported CTCP requests.
circe-ctcp-SOURCE-handler: add

CTCP SOURCE replies with the github URL by default.
circe-server-display-message: clean up CTCP display code

Pull out generic CTCP-displaying code, and be less repetitive when
displaying unknown CTCPs and responses. Also, actually use regex match
results where necessary.
circe-add-message-handler: make idempotent.

Closes #55.