Allow passing a function to align

[neoncube2]

Allows passing a function to align, to determine the alignment based off of the position, e.g.:

function getAlignment(position) {
  return position === 'bottom' ? 'end' : 'center';
}
<Popover
  isOpen={true}
  positions={['bottom', 'left']}
  align={getAlignment}
  content={<div>example</div>}
>
  Click me
</Popover>

See #206

[neoncube2]

@alexkatz Just a reminder about this PR, if you have time :)