Org babel R command session

[Sam <sam@crawf.uk>]

I just noticed an inconsistency between the way ob-R and ob-python
select executables. For python, there are 3 variables:
org-babel-python-command, org-babel-python-command-session and
org-babel-python-command-nonsession, with the former overriding the
latter two.
 
For R, there is only org-babel-R-command, which is ignored when running
as a session.
 
Is it worth trying to make the R functionality as similar as possible to
the python? If so then I'm happy to submit a patch doing just
that. However, I'm not sure what is the best way to handle giving
command-line arguments to the R interpreter when initialising a session,
because you need to set inferior-ess-program to the executable
specifically, and put any arguments in inferior-R-args.
 
Cheers,
Sam