Protected/Private repository support for seed cli command #142

Hi team,

We're thinking of setting up the stack via cli's seed command (includes creation of the stack as well).
While running this command, we noticed that it requires a release on a public repository to proceed, which may not always be the case in an size-able organization.
Thus, I request you to add support for authenticated github requests in the cli for seed command so that we can run it from a protected github repository as well.

Thanks!
~RG