Select options don't change focus when using arrow keys

[Subtletree]

When moving between select options with up and down keys, ember-power-select uses aria-current=true to show what option is focused by adding styles to .ember-power-select-option[aria-current=true].

Angular material uses the :focus selector to show what option is focused.

Since paper-select doesn't focus the options the focused option is never shown:

Angular's version:

If I was to put a pull request together for this, would it be preferable to:

1. Keep to spec and make ember-power-select focus the options.
2. Add the styles to [aria-current=true]