Add a groupByMulti template function

Supports the use case of having multiple values stored in an env
var that are separated by a comma or other char.  For example,
w/ nginx-proxy, VIRTUAL_HOST=foo.example.com controls the virtual
host that will be setup.  If an container supported multiple
virtual host, it could not be done w/
VIRTUAL_HOST=foo.example.com,bar.example.com.