use_oob
if FALSE, use a local webserver for the OAuth dance.
Otherwise, provide a URL to the user and prompt for a validation
code. Defaults to the of the "httr_oob_default" default,
or TRUE if httpuv is not installed.
as_header
If TRUE, the default, sends oauth in bearer header.
If FALSE, adds as parameter to url.
cache
A logical value or a string. TRUE means to cache
using the default cache file .oauth-httr, FALSE means
don't cache, and NA means to guess using some sensible heuristics.
A string mean use the specified p