• Add option version_directive (print %YAML 1.2)
  
• Add option yaml_version
  
• Add support for detecting the right schema depending on the
  YAML version directive
  
• Support both %YAML and %TAG directive for a document
  
• Improve emitter (output footer ... after open-ended block scalars)
  
• Improve emitter (quote strings starting with --- to reduce ambiguity
  for the reader)