Allow premium accounts to configure consumer_key and consumer_secret instead of bearer token

[tonybaloney]

Previous behaviour:
premium users have to specify bearer_token in their credential files

New behaviour:
premium users can specify bearer token as before
they can also use consumer_key and consumer_secret in the creds YAML file. If this is specified, the bearer_token will be set on startup by a helper function inside the credentials submodule.

[tonybaloney]

@binaryaaron

[binaryaaron]

Thank you for the PR! I was quite literally adding a bearer token utility to the library and command-line interface.

I like where this is going, but I'll leave some specific feedback soon.

[binaryaaron]

@tonybaloney - can you change the target of this PR to credential_updates? I'd like to integrate the bearer token functionality into the command-line app as an explicit command and will update the documentation as well.

[binaryaaron]

Hey @tonybaloney - I'd love to merge this if you could change the target branch to credential_updates when you get a chance. Thanks!

[tonybaloney]

@binaryaaron done

[binaryaaron]

Thanks!