Add 'controlsOnlyForSpeaker' option

[gpotter2]

Hi. Thanks for this great project.

This tiny PR adds a setting 'controlsOnlyForSpeaker' that only displays the controls for the presenter.

[jdeniau]

I'm not related to reveal.js, but I think that this could be a value for controls:

controls: boolean | 'speaker-only';

This way, we could implement 'display-only' of other kind of extension. Moreover, it avoid adding another prop to the API.

[hakimel]

I like @jdeniau's suggestion here. Can you update it so that this mode is enabled via controls: 'speaker-only'?