[feature request] match a set of hosts instead of all hosts

[maplebed]

In the callback mode, it'd be nice to specify a host substring or regex to match instead of triggering the callback on all hosts. Maybe you only need disk monitoring on your db hosts but you want to use the callback method. Right now, the responsibility lies in the person writing the callback to do host selection; it would be easier to write checks if it were clear how to pass that restriction in to the library.