feat: Allow mirroring the main slide

[mfornasa]

Add support for launching a secondary instance that mirrors the main instance.

Chunks are supported. Code selective highlighting is not supported at this time.

Closes #758

[mfontanini]

I'm a bit iffy about this for 2 reasons:

• This doesn't mirror everything. e.g. if you execute code in your slide (which includes things like a code block using +image) you won't see those changes in your mirrored terminal.
• This changes the meaning of --publish-speaker-notes to something entirely different.

In your comment in #758 you mention:

However, your suggestion to use tmux seems problematic, as #750 (comment), so title slide and headers are not rendered correctly.

Which is fair, but also is a problem here since you can't see executed code so you don't get the presentation mirrored 100% on either solution.

[mfornasa]

Which is fair, but also is a problem here since you can't see executed code so you don't get the presentation mirrored 100% on either solution.

Thanks for clarifying that, @mfontanini.

Do you have any ideas for a better solution that would also preserve the style?