extend taskopen to work with task "properties"
In an effort to expand the capabilities of task annotations by establishing a key: value pair convention, (as outlined in tw feature request #1088 - task "properties" or "casual attributes") and that has direct implications for taskopen. 

I'm encouraging changes to task open that will extend the current behaviour to annotations that have a leading key: <something>, so that annotations that start with

 goals: ~/foo-objectives.txt
 notes: Notes
 devsite: www.tasktools.org

would behave nicely, but with shiny new semantic labels. I would also be desirable to be able use those new labels as in "taskopen 10 goals". 
