message: tidy up Link; add propTypes
diff: import React from 'react'
  +import PropTypes from 'prop-types'
  import classnames from 'classnames'
  -const Link = ({children, className, muted, gray, graydark, nounderline, ...rest}) => (
  +export default function Link({children, className, muted, scheme, nounderline, ...rest}) {
  + return (
  <a
  - className={classnames(className, 'text-blue', {
  - 'muted-link': muted,
  - 'link-gray': gray,
  - 'link-gray-dark': graydark,
  - 'no-underline': nounderline
  - })}
  + className={classnames(
  + className,
  + 'text-blue',
  + muted && 'muted-link',
  + scheme && `link-${scheme}`,
  + nounderline && 'no-underline'
  + )}
  {...rest}
  >
  {children}
  </a>
  )
  +}
  -export default Link
  +Link.propTypes = {
  + children: PropTypes.node,
  + className: PropTypes.string,
  + href: PropTypes.string,
  + muted: PropTypes.bool,
  + nounderline: PropTypes.bool,
  + scheme: PropTypes.oneOf(['gray', 'gray-dark'])
  +}
  