Vim: giving up on VAM addon manager.

VAM (Vim Addon Manager) seemed like a good idea. I like the declarative
syntax and dependency management. But it just doesn't seem to work
reliably.
Vim: add Vundle to manage plugins.

Chose Vundle because it allows us to just declare the names of packages
in the Vim config files (and run `:PluginInstall`). This makes it much
easier to deal with plugins -- we can just add them to the config file,
commit to git, and run `:PluginInstall!`. We don't have to update our
installation scripts, just our config files.
Vim: add numbertoggle plugin.

This allows us to easily toggle between absolute and relative line numbers.
It will automatically switch to abolute line numbers in insert mode, and
relative line numbers in other modes.

Can also manually toggle with <leader>tn (my own key-binding).
Vim: add rainbow_parentheses.

Will automaticallt color each matching set of parentheses (as well as
braces and brackets) differently. This lets us easily see which open
parentheses match up with which closing parentheses.