googledrive 2.1.1

• drive_auth(subject =) is a new argument that can be used with
  drive_auth(path =), i.e. when using a service account. The path and
  subject arguments are ultimately processed by
  gargle::credentials_service_account() and support the use of a service
  account to impersonate a normal user (#413).

• All requests now route through gargle::request_retry() (#380).

• drive_scopes() is a new function to access scopes relevant to the Drive API.
  When called without arguments, drive_scopes() returns a named vector of
  scopes, where the names are the associated short aliases. drive_scopes() can
  also be called with a character vector; any element that's recognized as a
  short alias is replaced with the associated full scope (#430).

• Various internal changes to sync up with gargle v1.5.0.