Support _repr_mimebundle_

In addition to `_repr_*_` methods, IPython also defines `_repr_mimebundle_` protocol with higher precedence:
https://ipython.readthedocs.io/en/stable/config/integrating.html#MyObject._repr_mimebundle_

Implementing this may be important in terms of correctness. For example, a subclass may use `_repr_mimebundle_` to shadow `_repr_html_` of the parent class.