Add default CTCP handling

goirc will now respond to certain CTCP requests automatically, such as
TIME, VERSION, and PING. This will only happen if no other handler for
CTCP has been registered. If you register a CTCP handler and wish to
invoke goirc's default behavior, call conn.DefaultCTCPHandler(line).
Fix filtering of outgoing messages

It turns out filtering was completely broken if a bad character was
detected. Instead of filtering out the bad characters, it ended up
filtering the entire message down to "".

Remove \001 from the bad characters list. I have no idea how that got in
there, but \001 is used for CTCP and so obviously is valid.