Support passing elements in props

[nippur72]

React allows to pass entire elements in props but that is not possible with react-templates.

Consider for example the following template:

<Menu>
  <MenuItem>aubergine</MenuItem>
  <MenuItem>butternut squash</MenuItem>
  <MenuItem>clementine</MenuItem>
</Menu>

in react documentation they suggest to use a Context or alternatively:

Recall that you can also pass entire React components in props if you'd like to.

Now what I propose is that we create a special tag <props> to be used like this:

<Menu>
  <props ref="subitems">    
    <MenuItem>aubergine</MenuItem>
    <MenuItem>butternut squash</MenuItem>
    <MenuItem>clementine</MenuItem>
  </props>
</Menu>

meaning that the <MenuItem>s will be available as this.props.subitems.

[nippur72]

Closing this as it overlaps with <rt-template>.