Performs the interactive three-legged OAuth 1.0a flow: it opens a browser so you
can authorize the app, then exchanges the verifier for a durable access token. The
access token is cached on disk (under tools::R_user_dir()) so the browser only
opens once; subsequent sessions reuse the cached token.
Details
You normally do not need to call this directly: any function that hits an
OAuth-protected endpoint will trigger it automatically when no cached token is found.
As before, you only need to provide your app's consumer key and secret via the
RTUMBLR_TOKEN environment variable ("consumer_key;consumer_secret").
