Make cancelling of IRC messages in staff channel more flexible

I couldn't think of a way to effectively fix #10, so I decided to make it more flexible so that users can change the formatting without breaking cancelling IRC messages in the staff channel.

It adds a StringList to messages.yml with regex strings. Loops over all of them and if it matches, cancels it. Includes regex `^\[IRC\].+` as a default, for the default formatting.