Feature Request: Add wysiwyg html editor as input cell type

Currently, I am aware of Jupyter notebooks support for three CLI type interfaces to generate html output in the notebook:
1. cell magic ```%%html``` followed by raw html in the cell.
1. python html output:
```
from IPython.display import HTML
htmlstr = '

'
display(HTML(htmlstr))
```
3. markdown cells.

To improve ease of use and flatten the learning curve a bit, I think it would be a good idea to add a WYSIWYG html editor such as [https://github.com/tinymce/tinymce](tinymce) as an input cell option in addition to markdown. I think this should not be too hard, but am not familiar enough with the client server communication protocols to be sure. If this is relatively easy and somebody can point me in the right direction, I might be able to do this. I have done a significant amount of interface programming for Plone and Sagemath.

Start by reviewing the existing markdown-cell input path and the client-server communication protocols; the issue does not identify files or tests. Compare the proposed WYSIWYG input-cell behavior with the existing HTML, Python display, and markdown-cell workflows. Done means a WYSIWYG HTML input cell is available alongside markdown and produces the intended notebook HTML output.

Written by the indexing model from the issue text.