like shown here, ipython/jupyter has display hooks that can be registered with display formatters (e.g. png, html)

it would be nice if IRDisplay and IRKernel would work together so that

1. we define a S3 or S4 generic (e.g. jupyterdisplay.default)
2. IRDisplay defines it for some classes (e.g. jupyterdisplay.matrix)
3. users can override it