Simplify implementation

[wheresrhys]

Currently fetch-mock and jest are quite convolutedly entwined in this implementation. It could potentially be simplified with the following approach

1. Retain the requireActual usage (which now lives in fetch-mock anyway)
2. Document that users shoudl use this pattern:

jest.mock('node-fetch', jest.fn())
const fetch = require('node-fetch');
const fetchMock = require('fetch-mock-jest').sandbox();
fetch.mockImplementation(fetchMock)

3. fetch-mock-jest still adds extensions to jest, but that check to see if the mockImplementation is a fetch-mock instance, and error if not.

[wheresrhys]

Or maybe a pattern like require('fetch-mock-jest').inject(jest.fn())

Which sets up the mockImplementation etc

Which then may as well always be assumed on the import

jest.mock('node-fetch', require('fetch-mock-jest'))
const fetch = require('node-fetch');

Perhaps the key is to always use a sandbox(), and avoid the global mode of fetch-mock - leave it up to jest to do the stubbing.

Could make life a lot easier too by renaming .mock() to something else on the sandbox. respond()?