feat: support providing a list of allowed packages

[G-Rath]

I've tried to write a test, but couldn't get it to work - it doesn't seem to accept the flag value I try to set (I also tried passing in a whole custom flag instance).

@maratori let me know what you'd like to do.

Resolves #20

[maratori]

@G-Rath Thanks for contribution!
Small comments from my side.

[codecov]

Codecov Report

Merging #21 (0e3477c) into master (16bac89) will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #21   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           1      1           
  Lines          27     36    +9     
=====================================
+ Hits           27     36    +9     

Impacted Files	Coverage Δ
pkg/testpackage/testpackage.go	100% <100%> (ø)

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[maratori]

LGTM
@G-Rath Thanks for contribution
I'll prepare new release this week (so dependabot will update it in golangci-lint).