Adding close event

This is branched from my other PR about adding an optionsFiltered event (https://github.com/BrandwatchLtd/selleckt/pull/49) which at the time of writing is open, hence why there are a few commits here not directly related to the close event.

This PR adds a "close" event, which is triggered when selleckt's dropdown is closed - this makes it easy to hook on behaviour like having the whole selleckt disappear when it closes.