Add rbenv setup to zshrc

rbenv (https://github.com/sstephenson/rbenv) provides a reasonable way
to install multiple Ruby versions and use them with different projects.

We're using this at Causes, so I'm adding some configuration that checks
if rbenv is installed and performs appropriate setup.
Only set up dircolors on Linux

OSX does not know about `dircolors`; constrain this to Linux.
Alphabetize submodules
Use `jj` to exit Vim insert mode

I'm trying to avoid installing the weird kernel extensions that I used
to use on OSX that would remap caps-lock to escape, but this is making
using vim somewhat ungainly. I'm going to try this for a while and see
if using `jj` to exit insert mode feels okay.
Add script to link dotfiles into home directory

To keep this all easy, I have added the install script to the root, and
nested the actual files to be linked into the home directory inside a
`dotfiles` subdirectory.
Add a basic README