加入一个OnlyTaggedField选项

[neverlee]

目前是支持忽略tag为"-"的结构体项。
但有时结构体比较大，一个个去加会比较麻烦，不知道你们是否考虑接受增加这样一个选项。
这样生成的config就只处理带tag的项

[codecov]

Codecov Report

Merging #222 into master will decrease coverage by 0.08%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #222      +/-   ##
==========================================
- Coverage   83.79%   83.71%   -0.09%     
==========================================
  Files          42       42              
  Lines        5413     5416       +3     
==========================================
- Hits         4536     4534       -2     
- Misses        692      695       +3     
- Partials      185      187       +2

Impacted Files	Coverage Δ
feature_config.go	87.57% <100%> (+0.07%)	⬆️
feature_reflect_extension.go	77.87% <33.33%> (-0.7%)	⬇️
feature_iter_skip.go	75.29% <0%> (-3.53%)	⬇️

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