message: add bg prop to Text
diff: import React from 'react'
  import PropTypes from 'prop-types'
  import classnames from 'classnames'
  -import {color, fontSize, spacing} from './mappers'
  +import {bg, color, fontSize, spacing} from './mappers'
  import {composeWithPropTypes} from './props'
  -const mapProps = composeWithPropTypes(color, fontSize, spacing)
  +const mapProps = composeWithPropTypes(bg, color, fontSize, spacing)
  const Text = ({tag: Tag, children, fontWeight, lineHeight, mono, nowrap, ...rest}) => {
  const {className} = mapProps(rest)
  