Add fit to width/height zoom operations.

I thought about the optimal zoom business some more, and I realized that there are some kinds of patches (especially the wall-of-text variety, which is quite common in the docs) for which the fit-to-width/height options are in fact more useful than the general optimal zoom command. So I've added these now. Keyboard shortcuts are Cmd/Ctrl+Alt+w/h, but as usual feel free to change them if you have a better idea.