allow multiple auth scopes

[lawremi]

Sometimes we need to specify multiple auth scopes, like "read_only" and "compose". The gmail docs made it sound like compose implies read_only, but that does not appear to be true, in practice.

[jimhester]

Using match.arg(several.ok = TRUE) does run us afoul of https://bugs.r-project.org/bugzilla3/show_bug.cgi?id=16659 (typos are silently ignored as long as one valid input is found). Regardless this is a good change and you are correct about the scoping, thanks!