Add optional two-way binding for tracking open/closed status of individual accordion items

[valorekhov]

%subj%
The above allows for external code to track open status of individual items in the accordion to execute additional actions, e.g. loading of external data on item open.
It's a simple change to the zfAccordionItem directive adding to following to the scope configuration object.
scope: {
active: '=?'
},

[zurbchris]

Can you clean this up to only alter relevant individual files? We'll handle creating the /dist versions.

[soumak77]

@zurbchris @valorekhov

I'm going to add similar functionality to the zf-panel directive. I'll add it to other directives that would benefit with this change including the zf-accordion-item.

[soumak77]

I've implemented PR #740 to handle triggering active state messages for components that use the foundation sub/pub system to show/hide components. Accordions and Tabs do not use the foundation sub/pub system to handle changing active state of elements. In these cases, two-way binding of scope.active would have to be used to be notified of active state changes. The two-way bound value should be readonly until support is added to the Accordion and Tabs to allow changing active state outside of the directive.