Add functions to yank uri's/ids
Add actions to start and stop a task as in #1

It is possible to start and stop a specific task this way. This should may be
extended to allow several tasks, though I'm not sure of how useful that is.
Also, this suggests that we display which tasks are started. A nice syntax file
would be a good way to do this.
Skeleton of how to annotate using external tasks

As suggested in #1 it would be very useful to annotate a task with the results
of another unite source. To do this we can add an annotate action to the common
kind which is present in nearly all sources. I need to do something to allow for
selecting the task(s) to use by opening another unite window that can only
select tasks.
Pass context to tag source

Tag source will need the context eventually. This simply passes it along for
future work.
Add annotate action to all sources for #1

Being able to add an annotation to any task is a very useful feature. I've added
this functionality. It allows the selection of any number of items and adds them
to any number of tasks as an annotation.

To do this I added a new action to the common kind which most kinds inherit
from. This then activates a taskwarrior source to select tasks to annotate.
There are some limitations to this approach. For example, we don't use the same
source for normal task listing and for task selecting leading to an issue with
adding matchers and sorters and such. The matchers and sorters have to be added
to both or we end up not being able to filter as expected when selecting a task
to annotate. I also limit the action available when selecting to only annotate.
I think this is a good limitation however.