notes: reset timer on 'r' key press.

I used reveal.js together with notes plugin and was annoyed that timer started on opening notes.
Usually you first prepare presentation, together with notes window, and spend a moment before starting it.
So I think it's desirable to have a functionality to reset timer.
Another use case is when a few people speak one by one, with common shared slides, and would like to measure time separately.

Please review the change, I may improve it if needed!