Add HISTFILE to save zsh history

History file needs to be specified or otherwise the history would not be saved
over multiple terminal sessions.
[Source](http://zsh.sourceforge.net/FAQ/zshfaq03.html#321)
Don't set $EYRC

We don't use Engine Yard anymore.
Combine zlogin settings into zshrc

* easier when everything is just one file
* most (if not all) interactive shells are login shells and we likely want those
  settings for interactive shells
Remove leftover snippets variable
Also look for dotfiles-local

While trying out the approach outlined in Croaky's latest [blog post], I
realized that running `rcup` would not also pull updates from my local
extensions. If you clone your extensions as `dotfiles-local`, this
update will pick them up.

Non-existent directories are ignored by rcup, so it won't cause issues
if you don't have `dotfiles-local`.

[blog post]: http://robots.thoughtbot.com/manage-team-and-personal-dotfiles-together-with-rcm