ob-R.el: allow use of alternative graphics devices

ob-R.el: allow use of alternative graphics devices

* lisp/ob-R.el (org-babel-R-graphics-devices): New defvar.
(org-babel-R-construct-graphics-device-call): Use it instead of a
hard-coded list of graphics devices.

The `org-babel-R-graphics-devices' variable now holds the association
between graphics file types and R functions.  This allows users by
modifying this alist to choose to use e.g. the Cairo* functions from the
R package “Cairo” or one of several alternative devices for SVG
output (SVGAnnotation, RSVGTipsDevice).