Add prelude-last-buffer

Are you amenable to adding something similar to the following:

```
(defun prelude-last-buffer () "Switch to last open buffer"
  (interactive)
  (switch-to-buffer (other-buffer (current-buffer) 1)))
```
If you are, I'll open a pull request. If you have suggestions for a keybinding or better name, I'll take that into account in the PR.