Add support for keeping struct property values

[sebastian-walter]

Sometimes I want to keep the original value of a struct property. Therefore I added a tag keep which allows me to keep the initially set value. It still sets the value if it is the zero value of a field.

[codecov]

Codecov Report

Merging #64 into master will decrease coverage by 0.1%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #64      +/-   ##
==========================================
- Coverage   94.49%   94.39%   -0.11%     
==========================================
  Files          10       10              
  Lines         799      838      +39     
==========================================
+ Hits          755      791      +36     
- Misses         23       26       +3     
  Partials       21       21

Impacted Files	Coverage Δ
faker.go	89.68% <100%> (+0.3%)	⬆️

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[sebastian-walter]

@bxcodec Do you know when you have time to look at the PR?

[bxcodec]

Hi @sebastian-walter ,
Thank you for the PR, really appreciate it.
And I'm really sorry for the late reply. I'm so busy lately

Overall, it looks good to me.

[sebastian-walter]

Thanks!