Added Config.AllowOmitEmptyStruct

[millerlogic]

Optionally allow omitempty to apply to an empty (zero value) struct, disabled by default.
Useful when there are a lot of empty structs that are often not used. I am not a fan of using pointers just for this side effect.
I can add tests if there is interest.

[codecov]

Codecov Report

Merging #460 into master will decrease coverage by 0.14%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master     #460      +/-   ##
==========================================
- Coverage   86.61%   86.47%   -0.15%     
==========================================
  Files          41       41              
  Lines        5103     5109       +6     
==========================================
- Hits         4420     4418       -2     
- Misses        542      550       +8     
  Partials      141      141              

Impacted Files	Coverage Δ
reflect_struct_encoder.go	81.45% <55.55%> (-2.59%)	⬇️
config.go	89.41% <100.00%> (+0.05%)	⬆️
reflect_struct_decoder.go	84.55% <0.00%> (-0.54%)	⬇️

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