internal: Improve handling of taxonomy term slugification. This paves the way to bring slugification options like transliteration to PieCrust. This change mostly makes sure we use one-way slugification, which means, for serving/previewing, we need to slugify each page's terms to compare to the stuff captured from the URL. It also simplifies things a bit in the code.

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
        self.enabled = True

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

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