Expandable groups and accordions

Adds expandable group patterns, including an accordion-style option (only one open at a time).
I just realized that according to idiomatic.js [there's no need for the `=== true` when checking for truthiness](https://github.com/rwaldron/idiomatic.js/#cond) (re: the attribute). You can just do:

```js
... && $groupParent.data('accordion');
```