bake: Several bug taxonomy-related fixes for incorrect incremental bakes. * Improve how the baker processes taxonomy terms and figures out what needs to be re-baked or not. * Create bake entries for clean taxnomy terms so they're not deleted by an incremental bake. * Add more information to bake records. * Slugify taxonomy terms is now done by the route in one place. * Fix a bug where the cache key for invalidating rendered segments was not computed the same way as when the caching was done. * Fix how term combinations are passed around, rendered, printed, parsed, etc. (TODO: more word needed in the routing functions) * Expose to the template whether a taxonomy term is a combination or not. * Display term combinations better in the built-in theme. * Rename `route.taxonomy` to `route.taxonomy_name` to prevent confusion. * Add options to show bake records for previous bakes.

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()