Make summary mode sticky, when prev/next day links are clicked

I saw this [comment on reddit](http://www.reddit.com/r/perl/comments/16reyl/suggestions_for_a_learning_project/c7yxa47) by raiph. I have felt the same need in the past when I was trying to catch up with Perl 6 irc logs. So, raiph's comment made me look at the code, and it was pretty easy to make a fix. The changes are very minor.

Please note that these are untested changes. I suppose I'd have to install my own IRC server, MySQL server and a web server in order to run `ilbot` and test these changes. The effort involved in setting this up seemed more than I can spare my time at present, so I punted on doing this. If this is a problem, feel free to reject the pull request.