Pulled automatic subdomain function from nginx-vhost to separate plugin.

The auto-subdomain plugin automatically populates the app's `VHOST` file
with 'appname.rootdomain.tld' when a `$DOKKU_ROOT/VHOST` file is created
with 'rootdomain.tld' AND when the released app does NOT have an
existing non-empty `VHOST` file.

As discussed in #263, this plugin preserves all functionality of
automatic subdomains under the old nginx-vhost while being compatible
with the new multiple custom domain VHOST system. Helper messages have
been added and commented parts of code.
Remove backup/import of `$DOKKU_ROOT/VHOST` from 00_dokku-standard
plugin.

The auto-subdomains plugin now handles backup/import of
`$DOKKU_ROOT/VHOST`.