serve: Refactoring and fixes to be able to serve taxonomy pages. * Page sources' `findPagePath` is renamed to `findPageFactory`, so that it also returns source metadata. * Page refs now store possible hits more properly, and use the previous point to also store metadata. As a result, they can also return a proper factory. * Implement `findPageFactory` correctly in all built-in sources. * When the Chef server matches a taxonomy page, get the source metadata from the page ref in order to make a more proper page. * Make the `getRoute(s)` functions explicitely say if they want taxonomy routes or not.

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()