DisplayFormatter changes
========================

There was no official way to query or remove callbacks in the Formatter API.
To remedy this, the following methods are added to :class:`BaseFormatter`:

- ``lookup(instance)`` - return appropriate callback or a given object
- ``lookup_by_type(type_or_str)`` - return appropriate callback for a given type or ``'mod.name'`` type string
- ``pop(type_or_str)`` - remove a type (by type or string).
  Pass a second argument to avoid KeyError (like dict).

All of the above methods raise a KeyError if no match is found.

And the following methods are changed:

- ``for_type(type_or_str)`` - behaves the same as before, only adding support for ``'mod.name'``
  type strings in addition to plain types. This removes the need for ``for_type_by_name()``,
  but it remains for backward compatibility.