Convert slide deck into PDF with Puppeteer

[yhatt]

This PR will support converting Marp slide deck into PDF directly. We use a headless Chrome through Puppeteer.

marp-cli recognizes --pdf option or -o option with .pdf extension.

# --pdf option
npx @marp-team/marp-cli slide-deck.md --pdf

# -o option with .pdf extension
npx @marp-team/marp-cli slide-deck.md -o convert.pdf

We are using puppeteer-core package to avoid installing a new Chromium to your computer, so the PDF conversion will require Chrome that is installed by user.

Fix yhatt/marp#50, yhatt/marp#164, and yhatt/marp#222.

For yhatt/marp user: marp-cli is based on the new Marpit framework. You might have to update your Markdown written for the pre-released Marp.

[Naereen]

Great!

[stilvoid]

Nice!