FEAT: Add option to execute download notebooks

[mmcky]

It would be nice to have the option to pre-execute the download collection of notebooks that are generated by the downloadnb option.

@AakashGfude this should be pretty simple to add? The julia team would like this feature.

Lets use:

jupyter_download_nb_execute = True

with a default value of False

[mmcky]

@AakashGfude the Julia team would like this to be able to populate their live environments with pre-executed notebooks. It is best to use the download collection as they contain URL referenced images.

[mmcky]

This has been merged #269