Tweak RequestHandler / ResponseHandler interface

[izumin5210]

No description provided.

[codecov-io]

Codecov Report

Merging #13 into master will decrease coverage by 0.54%.
The diff coverage is 92.3%.

@@            Coverage Diff             @@
##           master      #13      +/-   ##
==========================================
- Coverage   86.77%   86.22%   -0.55%     
==========================================
  Files          11       11              
  Lines         378      392      +14     
==========================================
+ Hits          328      338      +10     
- Misses         31       33       +2     
- Partials       19       21       +2

Flag	Coverage Δ
#core	87.91% <92.3%> (-0.73%)	⬇️
#pb	77.04% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
option.go	88.59% <100%> (+2.42%)	⬆️
request_handler.go	100% <100%> (ø)	⬆️
response_handler.go	80.88% <100%> (ø)	⬆️
config.go	77.35% <63.63%> (-4.46%)	⬇️

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