fixed #178: the object opts_chunk_attr gives attributes of chunk options
fixed #178: the object opts_chunk_attr gives attributes of chunk options

fboehm/cboettig_site,fboehm/cboettig_site,fboehm/cboettig_site,fboehm/cboettig_site,fboehm/cboettig_site,fboehm/labnotebook,antass/labnotebook3,fboehm/labnotebook,fboehm/labnotebook,fboehm/labnotebook,fboehm/labnotebook,antass/labnotebook3,fboehm/labnotebook,antass/labnotebook3