Add Cypress adapter

[panmona]

It would be really cool to be able to use this tool together with Cypress as this is IMO currently the best tool out there to write tests in the browser.

There are other open source solutions for it already (for example https://github.com/jaredpalmer/cypress-image-snapshot) and many commercial ones. This one seems to be the better choice overall though.
For a list of currently available visual diff tools for Cypress see this: https://docs.cypress.io/plugins/directory#visual-testing

Would you consider adding a Cypress adapter to this library?

[NiGhTTraX]

Hey @panmau, yes Cypress support is on the horizon.

The implementation will be different from the existing adapters as the Cypress test runner executes in the browser, so Mugshot's Node dependent logic will have to be driven from a Cypress plugin.

I'll keep the issue open and come back once I'm done with this.