Visual regression tests

[0ubbe]

Following #13, we concluded it's worth adding visual regression test to this library as well as testing behavior in an actual browser so that we can refactor and update features with an extra level of confidence.

Cypress seems a good candidate for tooling, we already use it extensively in some products at Zopa and we're quite happy with its features and developer experience.

Any other suggestions welcome ❤

[poteirard]

Also, investigate if we could use https://github.com/styleguidist/snapguidist for this

Edit: Snapguidist doesn't support styled-components and CI