Move some Zsh config into zshenv file

Details from https://github.com/thoughtbot/dotfiles/commit/6a1bd18a1f20f8a384e5916a63e2b67d949eb02a:

From the [Zsh manual](http://zsh.sourceforge.net/Intro/intro_3.html):

> '.zshenv' is sourced on all invocations of the shell, unless the -f option
> is set. It should contain commands to set the command search path, plus
> other important environment variables. `.zshenv' should not contain commands
> that produce output or assume the shell is attached to a tty.

Why is this important? [Alfred](http://www.alfredapp.com/) workflows run in
non-interactive shells. When the `$PATH` is set, or `rbenv` is initialized, in
`zshrc` instead of `zshenv`, those workflows will not use the correct Ruby
version and might not have access to certain bin files, such as those from
`$HOME/.bin/` or Homebrew.
Flesh out the install instructions
Add notes about customizing the dotfiles