[Enhancement] Support absolute placement / positioning

[les2]

If the popover is inside an element with a overflow:hidden property over, then you cannot use it.

ember-power-select has a renderInPlace option that if false, causes it to use ember-wormhole in order to render its popover to a direct child of body.

[tim-evans]

Oh dear. I think it's reasonable to support this.

I specifically wanted this for conditional placement on mobile; your use case looks related to tight coupling of ember-power-select to select2, but that is what it is. PRs welcome for this.