feature rq: support speaker mode

[mmbcorley]

in Speaker mode (after pressing 's'), the console is only shown on one browser window (so much craning of neck to see what's being typed behind the speaker!). Is it possible for the console to duplicate, like slides, in this mode?

[georgestagg]

One thing that makes this difficult is that the console is interactive, and it's not so easy to send its entire state to the speaker mode window.

You can see a similar issue in the Reveal.js demo presentation at https://revealjs.com/?demo: Opening speaker mode and then navigating to slide 18 (Iframe Backgrounds), the main slides show an interactive iframe window but the speaker notes show only a black screen.