How do I create a widget for my armature I made a simple door with a one-bone armature that allows it to swing plus/minus 90 degrees, and I wanted to have a widget to grab while posing it, instead of using the bone itself. Something like arrows pointing in the direction of each axis. 



How would I assign such an object to the armature so that when in pose mode, I see and manipulate it instead of the bone?

I've seen UI in other rigs (Pitchipoy, etc.), but I can't tell how they're set up for this. There's a Custom Shape property in the bone panel, but it seems to replace the model for the bone itself, whereas I'm looking for a sort of proxy for that bone.
This could be one way to create a custom shape to control your bones: create a custom bone. https://youtu.be/6aRdurh30f0?t=1m9s

However, I'm sure you're more interested in this way, creating a custom shape that can control an aspect of a bone (like how the arrows in your image control the rotation of the door). https://youtu.be/8-IkdjzyJMQ?t=23m4s

Hope this helps!