Support Docker Credential Helpers

[lorenzleutgeb]

In addition to passing credentials as arguments, it would be very helpful if you could implement Docker Credential Helpers. That would greatly improve the usability of hocker.

[ixmatus]

@lorenzleutgeb, thanks for the issue and sorry for the extremely long delay in responding. For some reason I wasn't notified of this issue. I agree that using the idiomatic approach is much better.

I likely won't get around to addressing this until it becomes a pain point, though.