Add jest and simple test

[nlopin]

I've added Jest to the project.

There is an issue when you test components with external js libraries. For example, MarkdownPreview works with Raphael and its' extensions to draw graphs. I haven't figured out yet how to mock these stuff properly.

[nlopin]

One big issue I'm thinking about. Jest doesn't work with refs properties, but MarkdownPreview is the wrapper around iframe and we interact with that iframe via refs.

[sosukesuzuki]

I run yarn and yarn run jest, console shows the error like below.
please add jest-localstorage-mock.

[flaviusone]

Jest doesn't work with refs properties

Can you detail this? Maybe you are referring to react-test-renderer and ref mocking?

Did you consider adding enzyme?

Great job for adding Jest. It's a big and important step for this project! 🎖