Feature: Introduced antigen to manage zsh configuration

antigen (github.com/zsh-users/antigen) is a small set of functions that
help you manage your zsh plugins, much like vundle for vim.

All prior zsh configuration is now located in this repo:
github.com/mattjmorrison/zsh_config with one exception. The
zsh-syntax-highlighting plugin is now pulling from
github.com/zsh-users/zsh-syntax-highlighting.

The new .zshrc file will automatically install and set everything up on
a new machine when antigen is not already there.