add: [export] Introduction of a framing script
add: [export] Introduction of a framing script

- atm returning header, separator and footer for
  both stix 1 & 2 export
- will do the same for other export formats, as a
  centralized script taking the parameters needed
  for the format in subject and returning the
  corresponding header, footer and separator

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