Add sit/stand timer

[nimurphy]

Pomodroid is excellent. Sit/stand timer would be a useful addition, based off the existing logic so that say one focus round in a set is stand-up. It could use a separate timer but that would seem like overkill and a lot of work.

Happy to help out if someone can give me 10 mins, only vaguely familiar with Vue. As far as I can tell this would comprise:

• Timer-controller - logic to read config and change rendering to indicate a standing pomodoro
• Drawer-timer - add config setting for number of standing rounds, or if going fancy, a % of total time to stand
• Timer.js - state, presumably init