add run_cell, etc. utilities

[minrk]

We've reimplemented running a cell against a Jupyter kernel in Python in a few places (nbconvert --execute, a few others).

We should probably have a few utility functions (or methods) in this repo that can be re-used.

Cases:

1. run cell and capture all output
2. record that output in nbconvert
3. redisplay that output in IPython

[minrk]

This is prompted by the fact that I wrote yet another version of these for dask here.

It would be cool to also have a %load_ext jupyter_client that registers magics for running cells on remote kernels, as we did over there.

[dsblank]

Here is a working version using the function from Min's dask link:

https://github.com/dsblank/jupyter_client/blob/master/jupyter_client/runapp.py

For some reason, when using a python3 kernel, it occasionally hangs waiting for output. But other kernels seem to work fine.

[takluyver]

That looks ready for a PR.

[dsblank]

PR created.