ENH: Implement code execution at the Transform layer

[mmcky]

It would be nice to implement code execution at the transform layer of sphinx to have shared execution for the different targets html, coverage, download notebooks and code block level caching.

• [ ] http://docutils.sourceforge.net/docs/ref/transforms.html

Ideas and Notes:

1. perhaps a single RST file could be built using dask.delayed + joblib for code block level caching.

2. coverage: it isn't immediately obvious how to implement tests as if they are run in the same language kernel then the state will be different for cells that follow (in html workflow). May need to consider two task trees for coverage and other execution runs?