Add support for settings.RICHTEXT_FILTERS

RICHTEXT_FILTERS is a list of items that are valid for the RICHTEXT_FILTER setting.  The plural version takes precedence if it is available and non-empty.  Each item in RICHTEXT_FILTERS is applied in order.

An alias for the ``richtext_filter`` template filter has been added to match the plural nature of the new setting: ``richtext_filters`` simply calls on ``richtext_filter`` for its output.