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 the "point" of accordions is to have only one active at a time, but that's a somewhat limited view; the `multi_expand` option allows for greater flexibility for other use cases.

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...