Add JSON unmarshal for subkey inspect

[mikiquantum]

Fixes #80 and #83

[codecov]

Codecov Report

Merging #98 into master will increase coverage by 0.07%.
The diff coverage is 55.55%.

@@            Coverage Diff             @@
##           master      #98      +/-   ##
==========================================
+ Coverage   57.63%   57.70%   +0.07%     
==========================================
  Files          92       92              
  Lines        3118     3114       -4     
==========================================
  Hits         1797     1797              
+ Misses        964      962       -2     
+ Partials      357      355       -2     

Impacted Files	Coverage Δ
signature/signature.go	65.00% <55.55%> (+4.06%)	⬆️

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