Abstract getStyleRule

[joebourne]

This PR attempts to fix #117.

@MicheleBertoli I was not sure how you would want this utility to be exported. To stop it being run when calling import 'jest-styled-components' we could export it from its own file i.e. import getStyleRule from 'jest-styled-components/getStyleRule', but it's less intuitive for users.

Let me know what you think. Thanks.

[MicheleBertoli]

Thank you very much for working on this, @joebourne.
This looks good to me.

What about import {getStyleRule} from 'jest-styled-components/utils'?

[joebourne]

No problem, and thanks for reviewing @MicheleBertoli.

I've fixed up the import now.

[MicheleBertoli]

Hey @joebourne, thank you very much for your update.
I believe you need to create a "utils" folder which points to the function (see "native").
Otherwise this would be accessible via jest-styled-components/src/utils, right?

[joebourne]

Ah yes sorry @MicheleBertoli, the import is now coming from a new utils directory at the root level.

[joebourne]

Hi @MicheleBertoli, can I get an update on this please? Is there anything else you want fixing up?

[quantizor]

@joebourne if you wouldn't mind getting this fixed up against the new 7.x beta code I'd be interested to include the new functionality in 7.0 final :)

[joebourne]

@probablyup great! I've just fixed it up if you could take another look. Thanks.