Feature request: class support and stats

[mmerlone]

First things first: thank you for such a beautiful piece of code. Amazing!

I have a shared network where I define some classes to ease management, like this:

On dhcpd.conf:

# Class Desktop
class "clsDesktop" {
    match pick-first-value (option dhcp-client-identifier, hardware);
}

On an included file:

# Host boss-notebook
subclass "clsDesktop" 1:c4:6e:1f:ab:cd:ef;

Would be nice to have stats about how many devices are defined by class and how many have an active lease by class.

Side note, I define the clients on includes, so include support would be also nice.

Best regards.

[lukebarone]

This is references in my requested list of featuers, #35