Very handy addon. Despite that there are free desktop software doing annotations better, this addon has a killer feature: full page capture - few desktop programs can do that and definitely not as fast AFAIK. Great work!
BTW I do hope that the shape could be modified - currently if you drag a rectangle into a certain size and release mouse, you can't change its size again. It's etched on screen. Undo helps but modification would be better, especially if you want to modify it many steps later and do not want to undo all steps.