Doit.im would be the perfect task manager for me if I could add either a context to a tag to subtask. For example, say I have a project that includes a list of items to buy. I'd like the items to buy to be a list of subtasks and I'd like to put a context or tag next to each subtask that represents the store I plan to purchase the items from.

This way if I have multiple projects (which I do) I can just look at the 'HomeDepot' context or tag and see all the items I need to purchase for all my projects.