Move plugin specific code to _rplugin.vim files.
Prepare the files from ftplugin directory to be submitted to Vim as
official runtime files. Remove all code specific to the Vim-R-plugin
from the official files and put them in the corresponding _rplugin.vim
files which will only be distributed as part of the Vim-R-plugin. Note:
the ftplugin/rbrowser.vim and ftplugin/rdoc.vim files will not be
submitted to Vim because they are generated and used only by the plugin.