code: function parser( extname, fcn ) {
  	var ext = extname,
  		f;
  	if ( !isString( ext ) ) {
  		throw new TypeError( 'invalid input argument. Extension argument must be a primitive string. Value: `' + ext + '`.' );
  	}
  	if ( ext[ 0 ] !== '.' ) {
  		ext = '.' + ext;
  	}
  	ext = ext.toLowerCase();
  	if ( arguments.length === 1 ) {
  		f = parsers[ ext ];
  		if ( f === void 0 ) {
  			return null;
  		}
  		return f;
  	}
  	if ( !isFunction( fcn ) ) {
  		throw new TypeError( 'invalid input argument. Parser argument must be a function. Value: `' + fcn + '`.' );
  	}
  	parsers[ ext ] = fcn;
  }
docs: PARSER // 
  FUNCTION: parser( extname[, fcn] )
  If provided a parser, sets the parser assigned to a specified filename extension. Otherwise, returns a parser for the provided extension.
  
  @param {String} extname - filename extension
  @param {Function} [fcn] - parser
  @returns {Function|Null|Void} parser, null, or undefined