internal: Code reorganization to put less stuff in `sources.base`. Interfaces that sources can implement are in `sources.interfaces`. The default page source is in `sources.default`. The `SimplePageSource` is gone since most subclasses only wanted to do *some* stuff the same, but *lots* of stuff slightly different. I may have to revisit the code to extract exactly the code that's in common.

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()