Make interactive oauth authentication optional,  for automation use cases

A fairly simple change that allows gdrive/gdrive to be imported and used in automation apps, by making interactive oauth optional.

For example, my use cases are
- I'm using ansible to pre install the oauth token and download binaries on target machines.
- I'm writing a go based build and release tool currently, that optionally uploads to gdrive. Being able to detect that we don't have an existing token in place and skipping that step is much nicer than being prompted directly.