Add DoT

[MartinBrugnara]

Add support for requiring TLS for upstream DNS.

[codecov-io]

Codecov Report

Merging #11 (327b9de) into master (0627d28) will decrease coverage by 2.60%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #11      +/-   ##
==========================================
- Coverage   25.58%   22.98%   -2.61%     
==========================================
  Files           6        6              
  Lines         172      161      -11     
==========================================
- Hits           44       37       -7     
+ Misses        121      117       -4     
  Partials        7        7              

Impacted Files	Coverage Δ
dns.go	0.00% <0.00%> (ø)
blacklist.go	69.81% <0.00%> (-3.53%)	⬇️
main.go	0.00% <0.00%> (ø)
cache.go	0.00% <0.00%> (ø)
metrics.go	0.00% <0.00%> (ø)
environment.go	0.00% <0.00%> (ø)

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