(patch from yihui, part of #7555)
http://www.lyx.org/trac/ticket/7555#comment:18
This adds the new format "R/S code" with extension .R and a converter from .Rnw.
git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@39509 a592a061-630c-0410-9148-cb99ea01b6c8