Timer Attribute - with pomodoro support

[cldjr1]

Add a Timer attribute. Render a play/pause button on todos to activate it.

• It could be included simply inline as a key:value pair to maintain compatibility with other todo.txt apps. Like timer:HH_MM_SS
• A setting to allow multiple timers at the same time vs automatically pausing one if another is started?

If used on a todo with the Pomodoro attribute present: pm:#:

• Minor change to how the pm:# attribute works to allow pm:0 to be added to a todo.
• Clicking play/pause reveals a timer counting 25 minute interval
• At the end of 25m, the pm:# value is incremented, the timer attribute is removed, and a 5m cooldown timer appears (just in ui, the cooldown timer doesn't need to be written to the txt file.)