Allow setting lookup DNS/TXT function

[huicao]

This change can allow us cache the record to minimize latency

[codecov-io]

Codecov Report

Merging #25 into master will decrease coverage by 0.16%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #25      +/-   ##
==========================================
- Coverage   66.50%   66.34%   -0.17%     
==========================================
  Files           7        7              
  Lines         827      829       +2     
==========================================
  Hits          550      550              
- Misses        208      210       +2     
  Partials       69       69              

Impacted Files	Coverage Δ
dkim/query.go	29.16% <0.00%> (-0.84%)	⬇️

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[foxcpp]

Ref. #4 #10

[foxcpp]

Global variable seems to be a quick hack. I think having a VeriifyOptions structure similar to SignOptions is more useful and clean.

[huicao]

I agree. Would you mind to bring up your patch again? I think this is very useful.