add fullscreen button to all demo pages

[Geokureli]

most of these are unplayable on mobile without it

[ninjamuffin99]

the way I think I'll go about this is what I'll call the "itch.io" method (since it seems like that's what itch does!)

Which is to have a button on the site that is added to each demo page that fullscreens the iframe.

1. enable the demos to be resized (see PR change resizable="true" for html5 demos #336 )
2. adding button to resize the iframe on the haxeflixel.com side (see PR add fullscreen button to demos haxeflixel.com#259)

[ninjamuffin99]

this should be live at: https://dev.haxeflixel.com/demos/