[BUG] ensure compatibility with styled components 5.x

[andrewtpoe]

EXPECTED BEHAVIOR

What's the behavior you're expecting to see?

I've been using styled-components-modifiers with v5 since it was in alpha without issue, but it's worthwhile to go through it and ensure it doesn't violate any documented best practices. (I don't think it does).

Also need to add v5.x to the peer dependencies to remove a compatibility warning on install.

ACTUAL BEHAVIOR

What's actually happening instead?

Installing in an application using styled-components v5.x logs a warning in the console.

SUGGESTED SOLUTION

Do you have any feedback on how this problem should be solved?

Verify the code is fully functional.
Add the new version of styled components to the peer dependencies.

[andrewtpoe]

closed by #47