performance: Refactor how data is managed to reduce copying. * Make use of `collections.abc.Mapping` to better identify things that are supposed to look like dictionaries. * Instead of handling "overlay" of data in a dict tree in each different data object, make all objects `Mapping`s and handle merging at a higher level with the new `MergedMapping` object. * Since this new object is read-only, remove the need for deep-copying of app and page configurations. * Split data classes into separate modules.

PRIORITY_FIRST = -1
PRIORITY_NORMAL = 0
PRIORITY_LAST = 1


class Formatter(object):
    FORMAT_NAMES = None
    OUTPUT_FORMAT = None

    def __init__(self):
        self.priority = PRIORITY_NORMAL

    def initialize(self, app):
        self.app = app

    def render(self, format_name, txt):
        raise NotImplementedError()