message: add "selected" prop to UnderlineNavLink
diff: import React from 'react'
  import PropTypes from 'prop-types'
  import classnames from 'classnames'
  -import {ITEM_CLASS} from './UnderlineNav'
  +import {ITEM_CLASS, SELECTED_CLASS} from './UnderlineNav'
  import {mapWhitespaceProps} from './props'
  export default function UnderlineNavLink(props) {
  - const {children, className: defaultClassName, tag: Tag, ...rest} = mapWhitespaceProps(props)
  - const className = classnames(defaultClassName, ITEM_CLASS)
  + const {children, className: defaultClassName, selected, tag: Tag, ...rest} = mapWhitespaceProps(props)
  +
  + const className = classnames(defaultClassName, ITEM_CLASS, selected && SELECTED_CLASS)
  return (
  <Tag className={className} {...rest}>
  @@ -22,5 +23,6 @@ UnderlineNavLink.defaultProps = {
  UnderlineNavLink.propTypes = {
  children: PropTypes.node,
  className: PropTypes.string,
  + selected: PropTypes.bool,
  tag: PropTypes.oneOfType([PropTypes.string, PropTypes.func])
  }
  