Add attributes for ARIA accessibility

[Tarmil]

Accessible Rich Internet Applications (ARIA) is a set of attributes that define ways to make Web content and Web applications [...] more accessible to people with disabilities. For example, ARIA enables accessible navigation landmarks, JavaScript widgets, form hints and error messages, live content updates, and more.

I think we should add the following:

• A function attr.aria : name:string -> value:string -> Attr which would create an attribute aria-name="value".
• Functions for standard ARIA attributes such as attr.ariaHidden : value:string -> Attr which would create an attribute aria-hidden="value".