Snapshot ipynb outputs and patch them back into original files as image/png where approprite

[eliselavy]

• Uses nbconvert and puppeteer to execute notebooks and produce static images for all outputs
• open original ipynb files and patch the images back in where approprirate
• add a pre scan of the ipynb file to collect cell-id's of the subset of cells that should have snapshots added

In order to solve problem of big articles : https://github.com/C2DH/jdh-notebook/issues/103