Toggle the aria-hidden attribute when captions are toggled

I noticed the aria-hidden attribute wasn't changing when captions were toggled on/off. This adds that feature.

I'm basing this on the [W3C specs](http://www.w3.org/TR/wai-aria/states_and_properties#aria-hidden) that state:
>If an element is only visible after some user action, authors MUST set the aria-hidden attribute to true. When the element is presented, authors MUST set the aria-hidden attribute to false or remove the attribute, indicating that the element is visible.