Jest: Implement embedded Jest player

[hramos]

[aaronabramov]

this is already on the main page.
do we want it for every example?

[cpojer]

I think we want to add it to specific things in the documentation, like we could have an example snapshot test and some example failed/passing tests.

We could even have a tutorial inside of repl.it where you have 10 failing tests and you have to fix them and/or use the right matchers. @lacker do you have any thoughts about this? This could be awesome for teaching Jest.

[lacker]

I think that sort of tutorial would be pretty cool, especially for getting into advanced features like spies and asymmetric matchers and so on.

[aaronabramov]

so what would be the acceptance criteria for this task?

[lacker]

You guys talk like you are a testing framework