Use opt+f to go fullscreen

[ravanscafi]

I was having trouble with going fullscreen on firefox (the navbar would not go away on fullscreen, not sure why), so I created this logic for a presentation myself. I think it's a cool feature and could help others, so I created this PR.

I used a simple "polyfill" just in case, but I can change anything as desired, add more docs, etc :)

[nkuehn]

Great addition that I wanted to do myself, too to be able to get "real" fullscreen presentations in all situations.

I have tested it and it also works when two windows are open in presenter mode. If you want to polish, you could also catch the msRequestFullscreen vendor prefix and catch the case when none is found at all.

I can't merge though (not the maintainer).

[aszx87410]

@jxnblk I was wondering that is there any plan to merge this PR? This feature is very helpful and it's a very straightforward way to make it fullscreen. It would be great if this can be merged.