Allow nested fields in keep_fields plugin

[goshansmails]

Fixes #752

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 9 lines in your changes missing coverage. Please review.

Project coverage is 74.28%. Comparing base (b85c502) to head (77532e0).
Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
plugin/action/keep_fields/keep_fields.go	90.90%	6 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #753      +/-   ##
==========================================
+ Coverage   73.95%   74.28%   +0.33%     
==========================================
  Files         157      160       +3     
  Lines       16498    16789     +291     
==========================================
+ Hits        12201    12472     +271     
- Misses       3614     3628      +14     
- Partials      683      689       +6     

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