Allow for removal some widgets from notebook output

[mlucool]

Given a notebook that looks like:

# %%
from ipywidgets import Output
o = Output()
o

# %%
with o:
    print('hi')

It will save something like this to the notebook:

   "outputs": [
    {
     "data": {
      "application/vnd.jupyter.widget-view+json": {
       "model_id": "3bdcd203b4624d669a8b001aabf58416",
       "version_major": 2,
       "version_minor": 0
      },
      "text/plain": [
       "Output()"
      ]
     },
     "execution_count": 11,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],

We'd still like to be able to strip these unusable output areas (assuming widget state isn't autosaved). This is complex as vnd.jupyter.widget-view+json is used for all jupyter widgets. Ideally, it would be something like "if its an execute_result and data has type vnd.jupyter.widget-view+json, look at "text/plain" and evaluate a regex to determine if we should strip this".