Make the popover placement of header popover configurable

[lorefnon]

Hello, thanks for creating and maintaining this library. It is very feature rich and versatile.

I use the weekly calendar component in my app such that it is hinged to the bottom of a sidebar. But the issue is that the month selection popover always opens downwards causing it to get cut off.

I'd like to propose that we add a prop to make the positioning of this popover configurable.