fix filters marshal to include regex strings

[bwagner5]

Issue #, if available:
N/A

Description of changes:
allow-list and deny-list were the first two regex and more complex data types introduced to the filters struct. encoding/json does not properly return the string representation of regexp.Regexp which breaks the verbose filter output since it used json.Marshal. This PR implements a MarshalIndent on the Filters type.

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[codecov-commenter]

Codecov Report

Merging #17 into master will increase coverage by 0.20%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
+ Coverage   89.46%   89.66%   +0.20%     
==========================================
  Files           7        7              
  Lines         826      842      +16     
==========================================
+ Hits          739      755      +16     
  Misses         57       57              
  Partials       30       30              

Impacted Files	Coverage Δ
pkg/selector/types.go	100.00% <100.00%> (ø)

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[haugenj]

LGTM 👍