Automatically fields

[jiangz222]

fix #73

[codecov]

Codecov Report

Merging #78 into master will decrease coverage by 2.04%.
The diff coverage is 79.86%.

@@            Coverage Diff             @@
##           master      #78      +/-   ##
==========================================
- Coverage   91.50%   89.46%   -2.05%     
==========================================
  Files          12       13       +1     
  Lines         683      769      +86     
==========================================
+ Hits          625      688      +63     
- Misses         33       47      +14     
- Partials       25       34       +9     

Impacted Files	Coverage Δ
hook/hook.go	94.73% <ø> (-0.92%)	⬇️
collection.go	84.15% <73.52%> (-4.22%)	⬇️
field/field.go	78.04% <78.04%> (ø)
field/custom_field.go	96.55% <93.93%> (-3.45%)	⬇️
field/default_field.go	100.00% <100.00%> (ø)

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