message: add m + p utils to FlexItem
diff: import React from 'react'
  import PropTypes from 'prop-types'
  import classnames from 'classnames'
  +import {mapWhitespaceProps} from './props'
  -const FlexItem = ({tag: Tag = 'div', children, flexAuto, alignSelf}) => {
  - const classes = classnames({'flex-auto': flexAuto}, alignSelf && `flex-self-${alignSelf}`)
  +const FlexItem = (props) => {
  + const {tag: Tag = 'div', children, flexAuto, alignSelf, className} = mapWhitespaceProps(props)
  + const classes = classnames(className, {'flex-auto': flexAuto}, alignSelf && `flex-self-${alignSelf}`)
  return <Tag className={classes}>{children}</Tag>
  }
  