Support single-line comments in template literals

[jeddy3]

import styled from 'styled-components';
const Title = styled.h1`
  // single-line comment
`;

Gives:

3:3 error Unknown word (CssSyntaxError)

style components uses stylis behind the scenes, a preprocessor that supports single-line comments.

It'd be great if this PostCSS syntax can cope with single-line comments too.

[hudochenkov]

I think this issue should be in https://github.com/gucong3000/postcss-jsx, because we use postcss-jsx only in stylelint.