Plugin model

[CraigCav]

If we intend to support multiple sources for generating visual regression examples (markdown source, jest source, mdx source etc), we probably also need a plugin model to tackle cross cutting concerns.

For example, emotion builds up a global state of styles in document when calling css and injectGlobal that doesn't get reset between states. The Recipe project current wraps the markdown source to reset emotion's global state between test runs to ensure styles don't leak across tests, however this wrapper would be broadly applicable to any type of component source that provided emotion styled components. We should consider making plugins to support this kind of utility.