allow to filter optionally by hostgroup

First, thanks for creating this awesome gem!

I am able to get most information from gem APIs, but in this case I would like to get detailed Nagios checks filtered by hostgroup.

This PR adds additional parameter `hostgroup` to `site.service_status` to filter status details by hostgroup. I've tested on Nagios v3 and v4,  they do have this available in status detail view. 

To be more abstract, I think this method might be better renamed to `status(options)`, but renaming would cause API breakage for gem users - so i kept the same method name.