Whole mess of changes to .vimrc

I'm introducing a whole bunch of changes here. I'm not even going to
attempt to document what they all do. Some I came up with on my own.
Some I got by raiding @sjl's own .vimrc.
Add Clam

\o/ for awesome shell commands in Vim.
Add Gundo

Gundo makes browsing Vim's undo tree less bad.