Relative Due Triggers

#### To request a feature...

* Clearly describe the feature.

Being able to set relative due/wait/etc dates that trigger upon being unblocked and possibly other events.

* Clearly state the use case. We are only interested in use cases, do not waste time with implementation details or suggested syntax.

I use a GTD methodology and make a lot of projects that utilize many dependencies. Many of my projects involve a series of steps I need to do and at some point waiting for somebody to get back to me. I tend to set waiting to something like a few days, after which I'd like said task to end up in my task next so that I can be reminded that I need to followup with said person and see what's going on on their end. The issue is that if I set that specific task to `wait:1w`, it's 1week from the day I made it, not 1 week from the date it is ready and no longer blocked by dependencies. The same issue can be said for recurrances and I imagine anything else.