Accepts the current layer when the link is clicked.
The JSON value of the [up-accept] attribute becomes the overlay's
acceptance value.
Example
<a href='/users/5' up-accept='{ "id": 5 }'>Choose user #5</a>
Fallback for the root layer
The link's [href] will only be followed when this link is clicked in the root layer.
In an overlay the click event's default action is prevented.
You can also omit the [href] attribute to make a link that only works in overlays.
