message: clean up UnderlineNavLink, add whitespace props
diff: import React from 'react'
  import PropTypes from 'prop-types'
  import classnames from 'classnames'
  +import {ITEM_CLASS} from './UnderlineNav'
  +import {mapWhitespaceProps} from './props'
  export default function UnderlineNavLink(props) {
  - const {children, className: defaultClassName, tag: Tag = 'a', underline, ...rest} = props
  - const className = classnames('UnderlineNav-item', !underline && 'no-underline', 'px-3', defaultClassName)
  + const {children, className: defaultClassName, tag: Tag, ...rest} = mapWhitespaceProps(props)
  + const className = classnames(defaultClassName, ITEM_CLASS)
  return (
  - <Tag className={className} activeClassName="selected" {...rest}>
  + <Tag className={className} {...rest}>
  {children}
  </Tag>
  )
  }
  +UnderlineNavLink.defaultProps = {
  + tag: 'a'
  +}
  +
  UnderlineNavLink.propTypes = {
  children: PropTypes.node,
  className: PropTypes.string,
  - tag: PropTypes.oneOfType([PropTypes.string, PropTypes.func]),
  - underline: PropTypes.bool
  + tag: PropTypes.oneOfType([PropTypes.string, PropTypes.func])
  }
  