cli: enable running sub-commands

[shoenig]

Instead of merging #4 directly, it'd be nice to be able to incorporate such sub-tools into the donutdns binary itself - avoiding the large chunk of duplicate code.

As a first small command, I always wanted a way to check if a query would be blocked or not without going through the trouble of launching donutdns and using dig to send a query, etc.

Instead let us do something like

donutdns check <domain>

And the output would be something like one of

Blocked by default list exact match
Blocked by custom list exact match
Blocked by custom list suffix match
Allowed