new: [stix import] Marking parsing at attribute level
new: [stix import] Marking parsing at attribute level

- Made marking parsing function reusable for
  event & attribute levels

devnull-/MISP,FIRSTdotorg/MISP,devnull-/MISP,FloatingGhost/MISP,Rafiot/MISP,SteveClement/MISP,MISP/MISP,FIRSTdotorg/MISP,MISP/MISP,FloatingGhost/MISP,FloatingGhost/MISP,Rafiot/MISP,FIRSTdotorg/MISP,MISP/MISP,MISP/MISP,SteveClement/MISP,SteveClement/MISP,FIRSTdotorg/MISP,FloatingGhost/MISP,SteveClement/MISP,Rafiot/MISP,SteveClement/MISP,devnull-/MISP,MISP/MISP,Rafiot/MISP,devnull-/MISP