Add --progress-images to CLI

[kevinschaul]

Uses similar code to the web GUI for saving intermediate images.

Fixes #75
Fixes #99
Fixes #154
Fixes #224

[tildebyte]

Are you sure this fixes #75?

[blessedcoolant]

I did some testing on this. Here are my notes.

1. Shorten the switch to just -save_progress in my opinion. It'll be in sync with -save_orig that we already have.
2. Change -save_progress from being just a switch to actually taking step values from user. For example: let -save_progress 5 with 5 being the controller for the step count.
3. You need to rebase this on top of the development changes. A lot of refactoring was done. So you'll need to update your code.
4. Even saving an image per step, I did not see any drastic performance loss. It's pretty fast.

[lstein]

@kevinschaul, thank you so much for contributing this nice feature. As my collaborators have mentioned, it would be best if you could rebase this PR against the development branch. The code is evolving quickly, and over time it becomes increasingly difficult to merge in contributions. I hope you understand.

[blessedcoolant]

@kevinschaul @lstein I have a very nice implementation of this feature locally using -save_progress. I think I will make a new PR with that feature added to this and add kevin as the co-author.

[kevinschaul]

Hi all, apologies I was away from keyboard this weekend. Thanks for all the tips. I'm happy to close this and help get #694 merged instead.