Convert from nuget - support credential store

quite a lot changes here
notes:
- I decided to not use any syntax in paket.dependencies when credentials are saved in config file
- If the PR is merged, issue for adding credentials to config should be opened - in my implementation credentails to config are added only on convert-from-nuget
- New option for convert-from-nuget ``--migrate-creds`` - not sure about the naming here, feedback welcome
- union args parser doesn't let restrict the ``--migrate-creds`` mode value to a DU, so a exn is thrown if someone misspells it