Cache images created by `plot_tt()`

[J-Moravec]

Making a report is iterative process.

In my report, I have a bunch of images for ~ 300 cells.

Every single time I translate the report to HTML, I recreate some 300 images (and translate them into 64bit encoded string).

This is very wasteful.

I would like if:

a) If images for the requested inputs exist, they won't be recreated. b) If the 64bit encoding string for the requested inputs exists, they won't be recreated.

This won't only speedup the process when re-translating the table, but also prevents creating another sets of images when the table with images is simply printed.