TinyMCE plugin tweaking Markdown behaviour.

@param {Object} editor TinyMCE editor instance
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 );
}