Add command for listing channels

[akavlie]

RFC-1459:

4.2.6 List message

     Command: LIST
 Parameters: [<channel>{,<channel>} [<server>]]

 The list message is used to list channels and their topics.  If  the
 <channel>  parameter  is  used,  only  the  status  of  that  channel
 is displayed.  Private  channels  are  listed  (without  their
 topics)  as channel "Prv" unless the client generating the query is
 actually on that channel.  Likewise, secret channels are not listed

Also, most IRC clients show a user count for all users in the channel list. I don't see explicit support for that, but I suspect that it's done by running /names on every channel in the list, and counting the results.