Add a print stylesheet

[neilparikh]

Right now, when you try to print from the full preview view, you just see the first page of the preview, along with all the app UI.

[thesephist]

This is a good idea. Though it's not a priority for me, I'd be happy to see a print stylesheet added.

My offhand guess is it should be quite straightforward. There's a .preview-content DOM element that contains all the preview content, and in the print stylesheet we'd need to expose just the contents of that and nothing else.

There's currently no special styling applied to that element, but there's some styles applied to its children.

As mentioned above — not a personal priority, so I probably won't get to it immediately. But if anyone is inclined to take a stab at this (shouldn't require any recompiling anything, just CSS modifications) I welcome you to fork the repo and try it. Happy to answer questions!