notebook.R and non-R code

[gordonwoodhull]

rcloud.call.notebook is so old that we didn't consider languages other than R - we explicitly ignore Markdown cells and drop everything else on the floor.

It should run all cells regardless of language. Since we don't have marshaling of results between languages, it should return the result of the last R cell. (This means that cells might run after the result is computed but before it returned. I think this is okay.)