message: Add justifyContent and alignItems props for Flow
diff: import * as React from 'react'
  import { withStyles, WithStylesProps } from '../../../styles'
  +import { AlignItems, JustifyContent } from '../../grid/Grid/Grid'
  import { Spacing } from '../Spacing/Spacing'
  export interface FlowProps extends WithStylesProps {
  direction?: 'horizontal' | 'vertical'
  vSpacing?: number
  hSpacing?: number
  + alignItems?: AlignItems
  + justifyContent?: JustifyContent
  }
  @withStyles
  @@ -24,6 +27,8 @@ export class Flow extends React.PureComponent<FlowProps> {
  const styles = {
  flow: {
  margin: `0 ${-hSpacing / 2}rem`,
  + alignItems: this.props.alignItems,
  + justifyContent: this.props.justifyContent,
  },
  flowHorizontal: {
  display: 'flex',
  