Move some comments around
Add support for /topic
Try to parse other /CMD messages better

Based on another look at the IRC RFC (1459), most commands look like

CMD <TARGET> <... optional ...>

Since it's tough/impossible to know what the implicit target is for a
command, I'm just going to make the default implementation for commands
that I haven't added support for

CMD <...>

where the remaining parts of the message come from whatever is typed after

/cmd