Add support for multi_expand back to accordion.

I had already coded this up before I saw #3720, but since it's a small change, and it allows for much greater flexibility with accordions, I thought I'd send a pull request anyway.

Yes, I know it can be argued that the "point" of accordions is to have only one active at a time, but I believe that's a limited view. Nothing about the design of accordions prohibits multiple being open (I'd argue that's the point of tabs, and the point of accordions is the animation effects). In any case, the `multi_expand` option allows for greater flexibility with accordions.

And I know I can just code up some custom toggle buttons myself, but I can just code up custom accordion functionality too, so I'm not sure why that's more special than this. IMO, the primary benefit of this plugin is the structured/unified data model, and adding this option enables switching between types with a minor change.

If this is a go, I'll gladly write tests and docs too...