Add an option to not prompt the user for oauth authentication
interactively. This is a no-op for the cli, where the option
defaults to true.

Use Case: In automation apps/scripts where gdrive/gdrive is used
as a third party module (as opposed to a cli app). It is desirable
to not be prompted interactively. Instead return the error and let
the calling code handle it.
Actually use the option to disable the interactive prompt.
Merge pull request #17 from singhsaysdotcom/master

Make interactive oauth authentication optional,  for automation use cases