Add loglevel option

[x90nik]

At some point, I won't need the logs in the console or the output file.

Take a look at the tun2socks implementation.
They used Uber's zap package.

[debug|info|warn|error|silent]

[MutantPiggieGolem1]

+1
Using verbose gives far too much information, but I would like a log of each query made and which upstream was used (+if it was fallback).
(e.g. "Query for adguard.com handled by https://dns.adguard.com in 20ms", "Query for github.com handled by cache")
I would also like to know whenever the bootstrap is invoked.