more zsh tuning

[treznick]

This PR allows more customization of the oh-my-zsh configuration.

Using a ~/.zsh/local.zsh file you can do the following:

export ZSH_THEME=dracula
export ZSH_DISABLE_COMPFIX=true
unset plugins
plugins=(git osx vi-mode)

if you don't want to use the continuity theme

this also gitignores anything that's not the continuity theme in the ~/.oh-my-zsh-custom directory