`secret` sharing option

In the Plotly Python library, we replaced the `world_readable` argument with the `privacy` argument which allows for a third sharing option: "secret". This works by making a second `PATCH` request: https://github.com/plotly/plotly.py/blob/master/plotly/plotly/plotly.py#L1328-L1365

We should add this to the R library as well.
