Make the first slide convertible into image

[yhatt]

Make the first slide convertible into PNG and JPEG image by Puppeteer.

# Convert the first slide into PNG image
marp slide-deck.md --image png
marp slide-deck.md -o slide.png

# Convert the first slide into JPEG image
marp slide-deck.md --image jpeg
marp slide-deck.md --image jpg
marp slide-deck.md -o slide.jpg

It would be useful for creating Open Graph image that can specify with image global directive and --og-image option. This purpose is very similar to mdx-deck's screenshot command.

You might think to want to convert from the whole of slide deck into multiple images, like decktape's --screenshots option. We have to refactor the conversion logic to support outputting multiple files in order to realize that. So it is not yet supported in this PR.

ToDo

• Add tests
• Update README.md

[yhatt]

We filled tests about images.

When you are using macOS with Hi-DPI environment like Retina display, the output image converted via stable Chrome 72 would be cropped by a half resolution, and only remaining the left upper area. This bug is already fixed in Chrome Canary 74, so we have to mention this in the documentation.

Stable Chrome (72)	Chrome Canary (74)