OAuth permissions request flows

[stevenle]

At some point, Amagaki will probably require some sort of Google Sheets (or other Google APIs) access, and I want to propose from the start that ama should request the minimum amount of permissions necessary to do a particular task.

So if a user runs what is similar to grow preprocess -p my-google-sheet for example, then a readonly access token should be requested. But if at some point in the future user runs something like grow translations upload (where translations are stored in a Google Sheet), then ama should then request readwrite access. Ama will need a permissions system to handle this case.