Add useful git configurations.

Namely:
- fetching the pull-requests, as pr/:number
- add a `tree` alias, to visualize the git-log as a tree, showing the
  different branches side-by-side
Make tmux splits and windows open in the cwd.

This is rather ugly - I wish there was a way to simply configure it -,
but at least it's recorded now.
Tweak the vimrc and zshrc, and add todo.txt.

I'll improve a lot of these configs later.