Add hasAria() assertion

[Turbo87]

This could act as a shortcut for hasAttribute():

hasAria(name, value, message) {
  hasAttribute(`aria-${name}`, value, message);
}

@MelSumner thoughts?

[MelSumner]

I like it because it seems like it will help bring this more into the developer's conscious stream while they're writing tests. 👍

[scalvert]

I think this may have been what I was getting at with #99 (comment).

👍