Adding support for custom user agent

[stefanoj3]

Address #27

[codecov]

Codecov Report

Merging #30 into master will increase coverage by 0.36%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #30      +/-   ##
==========================================
+ Coverage   85.03%   85.39%   +0.36%     
==========================================
  Files          15       16       +1     
  Lines         481      493      +12     
==========================================
+ Hits          409      421      +12     
  Misses         41       41              
  Partials       31       31

Impacted Files	Coverage Δ
pkg/cmd/config.go	47.82% <100%> (+2.37%)	⬆️
pkg/scan/bootstrap.go	85.36% <100%> (ø)	⬆️
pkg/cmd/scan.go	84.41% <100%> (+1.31%)	⬆️
pkg/scan/doer.go	100% <100%> (ø)

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