Make setup function callable to reset mocks between tests

[trevoreyre]

This addresses #83

It makes the export of src/setup.js a callable function, so if you need to reset all of your Jest mocks between test runs, you can call the setup function again to reset your localStorage mock.

import { setup } from 'jest-localstorage-mock/dist/setup'

beforeEach(() => {
  setup()
})

afterEach(() => {
  jest.resetAllMocks()
}

The default export of the library will work the same as before. I'm not sure if this is exactly how you would like to resolve this issue, so I'm open to feedback. And I think the README.md would probably need to be updated to mention this option.