Add plugin loader script for antigen, oh-my-zsh et al

This makes using the sysadmin-util scripts repo plugin compatible so it's easier to use with ZSH frameworks like antigen and oh-my-zsh. For antigen in particular, adding them to your environment is simplified down to adding ```antigen plugin skx/syadmin-util``` to .zshrc, and antigen will then take care of cloning the repo and adding it to $PATH. This makes it a lot easier to keep tools in sync across multiple machines.