Adding a utility to help with managing lists

[wttw]

I found some sites I use being blocked and it wasn't easy to work out why.

This PR adds a small utility donut that can fetch all the blocklists in the default set, then search them to see why a hostname is blocked (e.g. it's on line X of the blocklist retrieved from URL Y).

It doesn't share any code with donutdns proper - there are one or two things it should share, like sources.json, but I copied them instead. So it could live in it's own repo, but it seemed to make more sense for it to be part of this one.

[shoenig]

Hi @wttw, this is awesome! Let me apologize for taking so long to respond. This is a feature I wanted pretty much since day 1, but in the back of my mind it felt like adding support for subcommands would be the way to go. But to make that work a bunch of refactoring needed to happen to make the list downloading stuff re-usable. In #16 we now have a rudimentary donutdns check subcommand - if you're still interested it'd be great to expand it with the details and caching behavior you've got in this PR.