Added new script vvv-new-site.rb

This script works a lot like the previous script, except that it will
now create WordPress multisite/network installations.

In addition, when specifying to use subdomains, it will create and
activate a plugin (vvv_host_update.php) that will automatically
update your vvv-hosts file.

This means that if you have vagrant-hostsupdater installed your new
sites will be added to the local /etc/hosts file (upon vagrant up,
resume or reload).