Support for recurring timers

[raghavkarol]

If you want to have a recurring timer, say every 5 seconds, you have to start a new timer every time it expires.

If this is too troublesome to do it will be possible to introduce recurring timers - I encourage users of Chronos to try it out as it is today and provide me with some information about how recurring timers should work for them. Then I will implement the feature.