Version 2.3.1

• Fixed PackageLoader. It now rejects fully qualified template names that could escape the package directory.
• Fixed the first and last filters when the Liquid.Environment class variable string_first_and_last is True. Setting the string_first_and_last option to True would correctly tell the .first and .last special properties to treat strings as sequences, but the same behavior was not implemented for equivalent filters. Now | first and | last behave the same as .first and .last. See #213.