Collapse: Support for pre/post collapse/expand callbacks

[nonplus]

In my application, before expanding a collapsed section, I need to populate the contents. Similarly, after collapsing an expanded section, I'd like to do some cleanup.

It would be great if the uib-collapse supported additional attributes that would be evaluated before and after the collapse animation, e.g. expanding, expanded, collapsing, collapsed. If the "before" attribute (collapsing/expanding) returns a promise, the directive should wait for it to resolve before beginning the collapse/expand animation.

Specifically, I'd like to be able to do something like this:

<div uib-collapse="isOpen" expanding="setupContent()" collapsed="cleanupContent()">
    <!-- complex & expensive content goes here -->
</div>

FWIW, I have an implementation and can create a PR if there is general interest in this...

[wesleycho]

I think this is a simple enough feature & should be simple enough to implement - I'm open to this.

[deeg]

@nonplus, do you have a PR you are going to put up, or should someone else look into implementing this?

[nonplus]

I need to write the unit test before providing a PR. Hopefully I can do that tonight.