Allow passing options to react-test-renderer .create() method

[jrdrg]

This PR adds the ability to pass options to the react-test-renderer create method, for example to allow mocking refs as per https://facebook.github.io/react/blog/2016/11/16/react-v15.4.0.html#mocking-refs-for-snapshot-testing

[jrdrg]

@arunoda @roonyh without this PR, running a snapshot test on a component that uses a ref will result in the following error:

[STRML]

This fixes #58 - would be very useful for us to deal with components that use refs.

[jrdrg]

@roonyh can you take a look when you get a chance?

[testerez]

With @jasonmorganson solution we could have a per-story mock: #58 (comment)
Maybe an even better solution would be to add it to story.options.storyshots.createNodeMock. It would be cleaner if we end up needing more configuration options...

That said, options passed to initStoryshots could still be usefull as a default.

[shilman]

Hey @jrdrg

Thank you so much for this PR! We're moving over to a mono-repo, since that makes a lot of sense for a project like this.

We would ❤️ it if you could create a PR here: https://github.com/storybooks/storybook

Please let me know if you have any q's and sorry for the hassle!