Reveals an element matching the given #hash anchor.
Other than the default behavior found in browsers, up.revealHash works with
multiple viewports and honors fixed elements obstructing the user's
view of the viewport.
When the page loads initially, this function is automatically called with the hash from
the current URL.
If no element matches the given #hash anchor, a resolved promise is returned.
Example
up.revealHash('#chapter2')
