Adding support for function-type saved datasets

[mgiamberardino]

Feature

The main idea is to be able to save named datasets accepting functions where you can create your datasets based on some extra data given by parameter when creating the fixture.

How to use it

fixtures.save('theName', ({ user }) => {
  const userId = user._id;
  return {
    entry: [{
      _id: '123456',
      user: userId,
      data: ...,
    }],
  };
});
...
fixtures({
  datasetName: 'theName',
  datasetExtras: { user: { _id: 'the_user_id' } },
}, .../* the usual callback */);

[kennethklee]

Sorry, I did not see this.

This is awesome! Can you add some documentation to the README.md and a test?