Support aria attributes

[eshtadc]

Ember still doesn't automatically associate aria- attributes in elements and solutions such as ember-component-attributes don't have a comfortable level of backwards compatibility.

Allowing ember-tether to bind the common aria- attributes would allow implementers to get a step closer to making more accessible applications.

I would propose just binding the global attributes as the tether element is a <div> if I understand correctly.

https://www.w3.org/TR/html-aria/#index-aria-global

(PR coming)