code: function markdown( editor ) {
  	function allowMarkdownAttribute( event ) {
  		const ed = event.target;
  		Object.keys( ed.schema.elements ).forEach( function( key ) {
  			ed.schema.elements[ key ].attributes.markdown = {};
  			ed.schema.elements[ key ].attributesOrder.push( 'markdown' );
  		} );
  	}
  
  	editor.on( 'preinit', allowMarkdownAttribute );
  }
docs: TinyMCE plugin tweaking Markdown behaviour.
  
  @param {Object} editor TinyMCE editor instance