Updates to free-sort plugin

[becky-gilbert]

These are the changes that @pjkohler made (#1018) and that I merged into this branch.

I made some additional changes:
-parameterized things like borders/colors/text
-added the option to display images in random locations inside the sort area, which was the old plugin behavior
-fixed the tests
-minor tweaks/updates to the docs
-more examples

I put this in PR because it's quite a lot of changes. I'd be grateful if @jodeleeuw @pjkohler or anyone else could have a look please!

[pjkohler]

Becky, how do I review this code?

[becky-gilbert]

@pjkohler But I was really just thinking that you could have a quick look through the file changes and comment here if you have any feedback. Or if you click on the 'Files changed' tab at the top of this pull request, I think Github also shows a 'Review changes' button that you can use to make comments. (Or maybe you can only see that button if I request a review from you first?)

And if you wanted to, you could also pull this code down locally and run the example file to see my changes. But I realize that might be a bit too time-consuming.

[pjkohler]

Looks pretty good to me.