Asynchronous onEnter and onLeave support

[craig0990]

This would be useful for manipulating elements with animations attached, such as waiting for a collapsible to slide down fully before highlighting.

Some basic ideas:

• onEnter and onLeave receive a done callback parameter, which is called with true or false
  • The plugin can check onEnter.length to see if it uses this argument, if it doesn't, it can be assumed to be synchronous (this is a convention copied from Mocha and other libs)
• or, done must always be called, which removes any ambiguity