Add optional number-of-days to history command.

Now, "/msg $bot history $channel $ndays" will get you back $ndays
worth of history. You're not limited to only two.

Note that $ndays refers to days on which somebody said something that
made a log line. If you have extended quiet periods in your channel,
$ndays=3 may get you things that are more than 3 days old. You'll get
logs from the last 3 days in which somebody spoke in the channel.