Allow a manual target to be set

[odejesush]

This change adds the ability to set a manual target that the tooltip
should be anchored to. A test is added to ensure that the target can be
set manually and that the tooltip is updated appropriately to be
anchored correctly.

Fixes #144

[danbeam]

@bicknellr dang it, I can't add source: hooli like i used to be able to. but just pretend ;).

[kanoni4567]

It'd be nice if the target isn't limited to siblings or direct parent.
It solves the problem of tooltip getting cut off when the target is inside any scrollable container, I can move the tooltip outside of the container and set the target manually.