Doesn't properly handle contextual selectors

[corbanbrook]

Causes errors to trigger when using contextual selectors like ${Button}

ie.

export const ButtonGroup = styled.div`
  white-space: nowrap;

  > ${Button} {
    margin: 0;
    border-radius: 0;
    vertical-align: top;
  }
`

149:28  ✖  Expected "$Button" to be "$button"     selector-type-case
 149:28  ✖  Unexpected unknown type selector       selector-type-no-unknown
            "$Button"

[ghost]

Thanks for the bug report! It would be great if you could submit a PR with a failing test for this. Otherwise I'll try to fix it myself asap.

[emilgoldsmith]

This issue is heavily related to #54, bordering a duplicate. Will definitely be fixed by the same PR when it happens.

Also the first linting error should no longer be a problem due to #59, but the second one for sure.

[emilgoldsmith]

This should be able to be handled with an sc-selector tag as added in #70 when we publish it in the near future (and maybe even inferred as proposed in #110 )