Cloudflare DNS support

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This also changes the default server to cloudflare, by decreasing the worker count on gdns to 0. That might not be optimal for consistency reasons. Someone could also combine the common parts of cfdns and gdns to to use the same / inherited structures.
I have very little experience with go(*this is the first go project i ever touched) and just needed this to work on my router as a static binary, so i'm not the best person to do that.

PS: Thanks you for your work. Have been using dingo for over a year with google and i'm very happy with it. :) PPS: I'm not so happy with google. I just happend to notice today that their server failed to resolve some domains and had to switch to another. I do hope cloudflare does fare better.