Upwards dropdown menu

The dropdown menu can now be set to be displayed on top of or under a button. Here's an example

// Set up dropdown
dropper.showWithAnimation(0.15, options: .Center, position: .Top, button: dropdown)

PS: This will not break your old code, because by default the dropdown is displayed by under the button.

[2.2] Bug Fix

Fixed issue where Dropper maintains previously set height (#1), thanks to sixfngers.