Add option to annotate screengrabs

Well, When I share screengrabs with my friends, I sometimes like to add annotations to them. I do this by pausing grabup, taking the screengrab as I would normally, opening it in preview and using the built in annotation tools before un-pausing grabup and taking a screengrab of my annotated screengrab, which is a bit of a pain.

My idea is to automatically open preview with the screengrab and then upload it once it has been saved, which would allow people to add their annotations. The main problem that I can see with this is how to turn this option on and off, after all, the reason most people love grabup is because of how simple and quick it is to use. I don't really have any ideas how to do this, apart from a key combination, but pressing 4 keys with one hand is quite hard, but then again, if its not an often used feature, it might not be that much of a trouble.