[Experimental] Support opening preview window provided by Carlo

[yhatt]

Support an experimental --preview option to open a preview window provided by Carlo. Marp CLI can open a mini window to show a preview of editing Markdown by using Chrome instance.

If you turned on --preview option, --watch option is enabled automatically too. It would provide a better experience while writing the Marp presentation in your favorite editor as like as Deckset.

Of course you can use it to the actual presentation by maximize window.

Restrictions

It is marked as an experimental support because of some restrictions.

• Marp CLI is still supporting Node v6, but Carlo is not supported, requires Node >= 7. You can not use --preview option if Marp CLI is running in Node 6.
• --preview is available only together with --server option. For regular conversion, we need to handle multiple windows against found files specified by file paths, glob, and --input-dir option.

Tasks

for this PR

• Add tests

In future

• --preview option for regular conversion
  • Handle multiple files
• -p alias
• Toggle some features
  • Fullscreen
  • Always on top (It might be impossible currently)