Add POST execute cell

[davidbrochart]

See jupyter-server/jupyter_server#900.

[davidbrochart]

@Waidhoferj I'm seeing this error with y-py v0.5.3:

File "/usr/share/miniconda/envs/jupyverse-dev/lib/python3.7/site-packages/jupyter_ydoc/ydoc.py", line 126, in set_ycell
    self._ycells.insert(txn, index, ycell)
y_py.MultipleIntegrationError: Cannot integrate a nested Ypy object because is already integrated into a YDoc: {'metadata': YMap({'trusted': False}), 'cell_type': 'code', 'source': YText(1 + 2), 'execution_count': 1, 'outputs': YArray([{'data': {'text/plain': ['3']}, 'execution_count': 1, 'metadata': {}, 'output_type': 'execute_result'}]), 'id': 'a7243792-6f06-4462-a6b5-7e9ec604348e'}

[davidbrochart]

@Waidhoferj I'm seeing this error with y-py v0.5.3

Fixed with y-py v0.5.4 😄