Synopsis:
   $topic(<#channel>)

Technical:
   This function returns what the client currently thinks the topic is.
   However during netsplits this information might get desync'd to what
   the client thinks. This is not a fault of the client.

Returns:
   topic of the channel specified or the current window current channel

Examples:
   $topic(#channel)       returns what the client thinks the topic is
   $topic()               returns topic for current window channel