Adds options for callbacks when Pushpin enters the different states: pinned, pin-top, pin-bottom.
I needed a way to trigger callback when the element was pinned, and "unpinned". Seeing I wasn't alone (#3685) I decided it to make a pullrequest instead of just monkey-patching my local version.
Adds optional callbacks for pushpin
Adds tests for pushpin callbacks
Updates documentation for Pushpin to include callback options
@acburst @Dogfalo Please look into it and do the needful