One-off feature

* Implemented as a timebox [1hr ago -> present]
* This will work seamlessly with Beeminder, where one-off events
    are recorded as datapoints of value 1
* Accessed through orwellize menu
* Just hit return to add a one-off item
* It was necessary to add a q for quit command (or use Ctrl-C)
* Use-case examples: Flossing, jogs, ...