Write demo output to the UI

[rviscomi]

Right now the demo writes capo.js results to the console. It's not totally natural to be using the demo web page and have to open DevTools to see the output so I think it'd be nicer if the results appeared in the page itself in a sort of emulated console UI.

One of the biggest challenges will be rendering HTML elements. Things like expand/collapse and syntax highlighting are free in DevTools but non-trivial to reimplement.

Maybe a tradeoff is to only print the color bars with tooltips, similar to the extension popup.

[rviscomi]

Design note: don't get XSS'd