This is based on the Token2.0 class provided in httr.
These objects should be created through the constructor function
gargle2.0_token(). In the base Token2.0 class, tokens are cached based on
endpoint, app, and scopes. For the
Gargle2.0 subclass, the identifier or
key is expanded to include the email address associated with the token. This
makes it easier to work with Google APIs with multiple identities. The
default location for the token cache is also different: it's now
"~/.R/gargle/gargle-oauth" instead of
An R6 class object.