feat: auto start rough notation animation on slide change

[mcanouil]

This PR adds an event listener to draw the rough notation on slide changes (see https://revealjs.com/events/).
(It keeps the key binding by naming the function and reusing it for both events, although with the auto start feature, the key binding is useless.)

Fixes #2

[EmilHvitfeldt]

I would use something like slide states instead of slidechanged, this way the user can do something like this to selectively apply it to slides

## No auto

## Yes auto {.rn-auto}

[mcanouil]

I see, this could also be a global yaml option with the implementation you suggest.

[mcanouil]

I am closing this PR 😉

[ChoCho66]

I want to ask a stupid question.
How to use this slidechanged function?
Or how to use this "auto start rough notation"?