d (even if empty) migrates the plugin to the new
system where the plugin object is kept across builds within one `mkdocs serve`.

Note that for initializing variables, the `__init__` method is still preferred.
For initializing per-build variables (and whenever in doubt), use the `on_config` event.

Args:
    command: the command that MkDocs was invoked with, e.g. "serve" for `mkdocs serve`.
    dirty: whether `--dirty` flag was passed.
Nr2