Add rich preview image when sharing the website on services like Slack, Whatsapp, etc.

[gianpaj]

Info about Open Graph:

• https://get.slack.help/hc/en-us/articles/204399343-Sharing-links-in-Slack
• https://developers.facebook.com/docs/sharing/best-practices#images

Test with this URL:
http://richpreview.com/?url=http://jasonpark.me/AlgorithmVisualizer/

I'm happy to do a pull request once you tell me what image should be used. Apparently some services even support gif, which might be useful to explain how the site works.

[64json]

Good idea, I think we can just use a screenshot of the webpage (without the frame of the browser) as the image. Your pull request will be appreciated 👍