Add presenter view to Bespoke template

[yhatt]

Add presenter view to Bespoke template by created Bespoke.js presenter plugin. Resolves #142.

Now we have the long-awaited feature! Presenter view for Marp presentation had strongly wanted in yhatt/marp#13 and yhatt/marp#30 from a lot of users.

It works with just a single HTML file in both of local and server. Thus, this feature does not break simplicity as a file converter.

[miguel-negrao]

I can report this branch works great on Linux Debian 10 with firefox 69.0 ! Really nice feature !

[asm0dey]

If this works fine — is there any chance that it will be merged in nearest future?

[asm0dey]

BTW works for like a charm after merging master into bespoke-presenter.

[yhatt]

This PR is still draft because of these:

• Timer is not yet work.
• We must fill tests for aiming to all green.

However, I've hesitated working because current plugin-based presenter view is feeling complex and lacked maintainability (As mentioned in our blog, Marp Next should keep simple coding for better maintainability).

Even so, I also could not think about better alternative proposal with supporting full features of HTML-based presentation provided by Marp / Marpit (Fragmented list, animations by CSS/GIF, interactivities by JS etc...). I'll work for this if contributors think this implementation was not so bad.

[asm0dey]

Thank you so much!