Add private registry capability

[Libbum]

Closes #102

It's geared directly at github, but I think it's general enough at this point (the known hosts step can be extended to gitlab or whatever if users request that). No credentials are stored anywhere, so no problems there. The PAT route isn't the best, but I doubt many organisations use that method.

[codecov[bot]]

Codecov Report

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  Hits              ?      364           
  Misses            ?        3           
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