Sandbox/Demo/REPL for testing on local and the website

[ylemkimon]

Currently, KaTeX has a demo at GitHub pages, which provides very basic demo, and a local test page on npm run, where some options are configurable using URL parameters.

It'd be useful to have KaTeX sandbox/demo for both local and the website with:

• GUI to configure KaTeX (GUI for test server (main.js) settings #1225, Add display/displayMode toggle on test page #1193)
• ability to run previous versions or test a PR. (Online screenshot diff/generation using CircleCI #1187, Update demo page to use v0.9.0. #1171, Add test job to CircleCI #1235)

There are many existing similar services and it'd be nice to adopt existing framework, but I can't seem to find a suitable one:

• CodePen
• JSFiddle
• Babel REPL
• CodeSandbox
• Ace Kitchen Sink

[kevinbarabash]

We might want to look at revising our gh-pages to have multiple pages. The current demo has both a small text input box and a small render output. In another issue it was mentioned having a way to also change the settings. I think a dropdown with specific examples would be nice too.