new format: TODO/DONE syntax for taskwarrior

[rohieb]

Now that each TODO/DONE item has a (user-supplied, but hopefully still unique) ID, we can extract those items and generate a synchronisation script that feeds the items into taskwarrior, or updates the state of existing tasks accordingly.

• come up with a way to generate taskwarrior UUIDs from user-supplied IDs
• find out how to address a single item by its UUID/user-supplied ID over the task interface without writing to taskwarrior's JSON directly