snapshot testing

[kokes]

seems like it will be doable via jsdom

to resolve:

• rename tests to test_cases?
• pin marked, katex, prism to the same versions as those CDNed (maybe in a separate PR, this will require testing)
• clean up the constructor in nbv.js
• make the HTML version into a working one
• module.exports won't work in the browser, find a workaround
• set up CI, add more examples etc.?
• throw exceptions instead of console.errors
• do not style the main div
• upload more test cases here
• walk directory with test cases to test them all
• fix katex errors

[kokes]

I downloaded a sample from this piece of research by JetBrains link, it will be useful for testing.

It's a public S3 bucket, so it was as easy as aws s3 sync s3://github-notebooks-update1/ data/ (need to Control-C it, there's a lot of data)