Support for specifying multiple servers

[nadiamoe]

Hello,

First of all, let me thank you for your great work with dingo, I'm currently finding it quite useful!

I was wondering if you had any plans of adding multiple server support for dingo. Something along the lines of what dnsmasq does, but simpler. The idea would be allowing the user to specify multiple servers, and one strategy for which one to use: Query all of them and return the fastest response, random/round robin, or fallback on error.

I can probably work on a PR implementing this if you find it interesting.