[Feature request]: Make the script generate .html files upon request

[inl-pd-autotest]

Current approach when .html files are generated at start - is suitable only for static galleries.

I'd like to be able to start the gallery script once and add/remove folders with images on the go, without the need to restart the script.

I failed to figure out how to modify the script so that it would work the way I want, so please, add this feature.

Maybe this needs to be a different script.