title: Stretch child to 100% width and height without changing the child's rules
body: <p>When parent is <code>display: flex;</code>, is there a way to force it's children to stretch itself to 100% of the parent's width and height, without accessing the children's props?</p>
  
  <p>i've found <code>align-items: stretch;</code> for vertical stretching, but can't find something for the horizontal.</p>