Merge pull request #4561 from minrk/for_type_clear Copied some of the logic from apptools (thanks @rkern). The current behavior: - `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 - `for_type(type_or_str)` - behaves the same, only adding support for type strings for consistency - `pop(type_or_str[, default])` - remove a type (by type or string) - `typ_or_str in Formatter` checks whether a type is registered, checking with `lookup_by_type` The inner structures remain unchanged.