Add support for Output widgets

[captainsafia]

Context: Found this while going through the Output Widget.ipynb example notebook.

Currently, if an Output widget is initialized, stdout and stderr streams are not redirected to it.

[miduncan]

Are you able to get the Output widget to render? I get an issue where it tries to access a library that we don't have in the project, I think like jupyter-widget/output or something. This part looks like an easy enough fix but I wanted to check that you hadn't done this already in an unmerged PR.

[captainsafia]

Yeah, this is what I'm working on at the moment. This and parts of nteract/nteract#4645 as well as parts of nteract/nteract#4648 are related to this.

The Output widget has to be implemented by us and needs to hook into our execute epics and state to redirect outputs correctly.

[captainsafia]

Updating the name of this issue and transferring it to the outputs package where the jupyter-widgets transform leaves now.

I started taking a look at implementing this in the fall and it was....messy. An output widget is essentially a transform within a Jupyter widget. Adding support for this in a way that is maintainable and easy to reason about is challenging for a couple of reasons:

• The Output widget usurps the standard output handling of the application it is in
• We have to re-implement support for our output rendering logic in the Output widget somehow

I started this work in nteract/nteract#4679 but tabled it after realizing how difficult it was to integrate into our package system.