Add break and resume commands

[soywod]

start and stop need the id of the targeted task. It could be nice to have 2 other commands that don't need the task id, and affect the last task used. Here the workflow I have in mind:

• I start the work day by unfog start id
• I decide to take a coffee, so I just unfog break
• I take back the work with unfog resume
• I finish my task with unfog stop id

The second benefit of those 2 new commands is that they can be used by other programs (since they don't require task ids). For eg, with a pomodoro tool, each cycle of break / resume can be easily hooked on them.