No longer use R to open the PDF from Rnoweb.
To get a better control of the PDF viewer, use Vim to open the PDF
generated by either Rnoweb or Rmd.
Changes:
  - The option vimrplugin_openpdf_quietly was deprecated.
  - New option: vimrplugin_pdfviewer.
  - The meaning of vimrplugin_synctex was changed.