Doesn't transform anonymous default exports

[kirstymullen]

The transformer doesn't include anonymous exports:
export default styled(Thing)

It might be best practice to ensure all exports are named, but I'm working in a large project that already has a significant amount of these unfortunately.

It looks like the plugin did support this at some point, as getDisplayNameFromNode is already able to handle it.

[kirstymullen]

I've created this PR to fix it: #367