[Request] Add a direction prop to set which way the picker opens

This is a new feature request. If you position the Datetime control towards the bottom of the page, the picker will appear below it and off the page entirely, forcing the user to scroll before they can make a selection. I hope it would not be too much trouble to add a `direction` prop which could take `oneOf(['up', 'down', 'left', 'right'])` and position the picker in that direction relative to the input.

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