feat: Support enhancing React component serializer

[alistairjcbrown]

snapshot-diff provides a convenience serializer for React components
which renders them and serializes them before diffing. Support is
already in place for allowing custom serializers to be added which will
work at the root level; serializing unknown types (e.g. React component
rendered with Enzyme).

However, there is no ability to update the serializers used within the
React component serializer. The use case for this need to when a
CSS-in-JS solution, such as Emotion, is used and custom serilization
is required to support outputting the styles attached. Emotion provides
this serializer, but adding at the root level then only outputs the
styles and does not output the component.

To support this, the same defaultSerializers and setSerializers API
as provided by snapshot-diff for adding root level serializers has
been applied to the React component serializer to allow
"sub-serializers" to be added which will then be passed into
pretty-format. This then provides an API which will allow the Emotion
serializer to work as expected with the rest of the React component
serialization process.

Fixes #162

[thymikee]

Mind rebasing? :)

[alistairjcbrown]

@thymikee Apologies, I'd forgotten about this PR - that should be it rebased again and I've done another run through the readme changes to try and make them a little clearer 🤞
If you have time, let me know what you think of the approach and, if it's ok, if there's any changes you'd like made 🙇

[oleksii-kachura]

This change would be nice to have, it would allow me to have diffs consistent with the jest-styled-components snapshots

import snapshotDiff from 'snapshot-diff';
import { styleSheetSerializer } from 'jest-styled-components/serializer';

snapshotDiff.reactSerializer.setSerializers([
  styleSheetSerializer,
  ...snapshotDiff.reactSerializer.defaultSerializers
]);