display leaflet in revealjs

[kazutan]

I love the revealjs package and I always use it.
However, using leaflet with revealjs, MapTiles may not be displayed properly.
That's the cause the style .reveal img {max-height: 95%} .

So, I add styles into default.html to fix this.

[cderv]

It is possible that this is fixed with a new version of revealjs - From quick test, it seems so.
I'll check then merge this is not.

[atusy]

@cderv Thank you for taking care of this. I think the fix on revealjs side is better than nothing for now.
Yet, as I mentioned in rstudio/leaflet#668, I wish in the future the fix happens on leaflet side because the same problem may occur outside revealjs, too.