Move my zsh theme to its own zsh-custom repo

* Replace my oh-my-zsh fork submodule with the oh-my-zsh master repo
  (https://github.com/robbyrussell/oh-my-zsh).
* Add my zsh-custom repo as a submodule
  (https://github.com/gg/zsh-custom).
* Update .zshrc to use $ZSH_CUSTOM.
* Update .zshrc to work with latest version of oh-my-zsh.
Fix sudo alias to work with 'nocorrect' commands

sudo would fail when used with certain commands that were prefixed by
'nocorrect', i.e.::
  # alias mkdir='nocorrect mkdir'
  $ sudo mkdir foo
  sudo: nocorrect: command not found