pfe-accordion: Add ability to direct link / open specific accordions

[starryeyez024]

It is a desirable feature to be able to link directly to a specific accordion in a set.

You could do this two ways, by URL, or by attribute. Assuming you have 3 accordions, Larry, Curly and Moe, you could open the 3rd accordion like this:

<pfe-accordion selected-index="3">

And/or by adding support for pfe-tabs-history attribute to pfe-accordion (which would be great because pfe-content-set can also upgrade to accordions). So a URL like redhat.com/my-page?tabs=moe should also open the 3rd accordion.

[dcaryll]

+1, would help on training pages where you need an active accordion on mobile (like Free courses)

[kylebuch8]

To see how we're already doing this with tabs on page load, review this code from tabs.

In the _init method, we're checking to see if there's something in the URL.
https://github.com/patternfly/patternfly-elements/blob/master/elements/pfe-tabs/src/pfe-tabs.js#L259

_init calls this:
https://github.com/patternfly/patternfly-elements/blob/master/elements/pfe-tabs/src/pfe-tabs.js#L454

Then if you follow the logic in _init, you'll see how we open a tab if we found a match
https://github.com/patternfly/patternfly-elements/blob/master/elements/pfe-tabs/src/pfe-tabs.js#L268

[starryeyez024]

Related issue #350

[castastrophe]

Any opinion about how we handle multiple open accordions?