Enable passing of href with options

Popups attaches to a elements with href attributes. This makes sense for the primary use case of adding popup behaviours to existing links. However, we may wish to provide JS-only behaviours not specifically linked to existing a elements (or linked to a elements but with distinct href attributes needed). Sample use case: being able to click on an existing node title to bring up an edit form for that node title.

Attached patch adds the option of feeding in an explicit href with the options argument. This is used if present; if not, we default to an href attribute of the element being processed. So we can attach popup behaviors to any element.