Option to use Vim wd as R wd

I've added an option to start R with a working directory that is vim's working directory rather than the directory of the file in the current buffer.  I think this would be a nice option to have especially since there is a command (\rd) to set the directory to that of the file you are editing.

I think this is especially convenient if you are working on a package and want R and Vim to be in the package/ directory rather than the package/R/ directory.

Thanks for taking a look.