Reconsider experimental preview option

[yhatt]

Unfortunately, Carlo, the base technology of experimental --preview option, is no longer maintained. (GoogleChromeLabs/carlo#163 (comment))

In a point of view from tool's maintainabillity, this fact may become a risk for us. So we have to reconsider whether must keep experimental --preview option.

Available options:

1. Continue using unmaintained Carlo
2. Switch to well-maintained Carlo forked by community or us
3. Stop using Carlo, and re-implement --preview option with puppeteer-core (We have already used puppeteer-core for slide conversion)
4. Stop using Carlo, and remove experimental --preview option

UPDATE: Took option 3 at #232.

[yhatt]

In Chrome 81 that is going to release in next tuesday, --preview option won't work because of not updated permissions in unmaintained carlo. We have already received some reports about this in #190 and yhatt/marp-cli-example#15.

At the moment we have not a time for maintaining forked carlo. In the next release, we will hide --preview option from help and output an error with keeping the original logic if using --preview option.

UPDATE: Upcoming releases for Chrome >= 81 was paused due to adjusted schedules probably affected by COVID-19. Luckily we are getting time to deprecate preview option during the stable Chrome is still 80.

[yhatt]

The delayed Chrome 81 was released at April 7th, and no longer work --preview option with it.

For a while I'll track community whether created well-maintained alternative of carlo, but we may have to decide removing --preview if not created.