MISSING THE BOAT TO LAKE WOBEGON: I may have swerved rightward in the past year or so, but I'm still a Garry Trudeau fan and I think Al Franken is damn funny. So when I read that the pair had dinner with Garrison Keillor last December in a kind of lefty humorists summit conference, I found myself wondering something I've wondered often in the past: Have I completely missed the boat on Keillor? Am I too young for Keillor? Is that it? Or does he have fans under forty? Or he is just not that great, a kind of Larry King or Jay Leno in his field, wildly popular for no discernible reason? Anybody out there read him and like him? What am I missing?