ChannelStats: enable Topic by default.

If the channel is not mode `+s`, random stats getting bots notice it, join there and publish the topic anyway. Examples:

* http://irc.netsplit.de/channels/details.php?room=%23limnoria&net=freenode
* http://irc.netsplit.de/channels/details.php?room=%23supybot&net=freenode