Add plist installer from nginx formula to allow launching from launchd

I added the plist installed from the nginx formula in homebrew to allow
launchctl/launchd to manage the startup of openresty.

One nice side-effect of this is that it's easy to setup running as root
to listen on port 80:

    sudo ln -sfv /usr/local/opt/openresty/*.plist /Library/LaunchDaemons
    sudo chown root:wheel /usr/local/opt/openresty/*.plist
    sudo launchctl load -w /Library/LaunchDaemons/homebrew.mxcl.openresty.plist