code: function isCustomFrontMatter( attr ) {
  
  	if ( attr !== 'title' && attr !== 'category' && attr !== 'content' &&
  		attr !== 'created' && attr !== 'excerpt' && attr !== 'updated' ) {
  
  		return true;
  
  	} else {
  
  		return false;
  	}
  }
docs: Check if front matter attribute is custom or not
  
  @param {string} attr The attribute name
  @returns {boolean}