Add StyledComponent to allowed Interpolation types

[mvestergaard]

This fixes an error when referencing another StyledComponent from within
a template interpolation string

Fixes #625

[mvestergaard]

I think I was a little too quick in assuming this fix works. This seems to cause props to have implicit any type in other scenarios. 😟

[mvestergaard]

I took a different approach.

I added a simple "placeholder" interface called ComponentRef that StyledComponent extends, and use that as a valid Interpolation value.

This seems to work, and does not break existing typings.

[pluma]

Is there anything missing from this PR to get it merged?

[mvestergaard]

A "problem" with this change, is that this will be allowed by the typing:

const NotStyled: React.SFC = () => <div />;

const Parent = styled.div`
  ${NotStyled} {
    color: green;
  }
`;

However personally I would rather that the typing allowed intended behavior, without having to use an escape hatch, than catching unintended usage.

[tkh44]

LGTM, but I'd rather have someone more confident in TS confirm to merge.

[pluma]

Awesome! 🎉