Remove .extend transformations as they've been removed from styled v4.

[Bdthomson]

.extend was removed from styled-components in v4 (styled-components/styled-components#1908),

This PR removes support for it in this plugin so it no longer has to be maintained, fixing issues similar #33.

TODO

• Cleaning up tests (if maintainer will accept the change)

Fixes #48

[Igorbek]

@Bdthomson thank you for the contribution!
I suggest adding extend to the Options.CustomStyledIdentifiers. And make the default value an empty array. In this case, if anybody wants to keep existing extends they can define it in the options.

[Igorbek]

I'm closing as it's been superseded by #370