Feature Request: Set origin mid-ruler

[Pigankle]

This is a great free ruler - thank you for building.

Feature Request: Ad the ability to set the origin ("0") of the ruler to somewhere in the middle. This would be useful for measuring to see if two elements are the same distance from an element in the middle. I imagine this works something like hovering over on the ruler and pressing the 0 (zero) key. Then the ruler will display 0 at that points, and count in both directions away from the new origin point.