Add a <yield> tag for transclusion

[nippur72]

To extend the concept introduced in #103 we could add a <yield> tag similar to that in Riot or Ember:

• <yield to="name">...</yield> would pass the element tree in this.props.name
• <yield from="name"/> would render the tree passed in this.props.name
• <yield /> with no attributes would be simply a shortcut for this.props.children
• <yield render="{this.someFunc()}"/> would render the tree returned from this.someFunc()

Some examples:

<my-splitter>
   <yield to="leftpanel"><div>hello</div></yield>
   <yield to="rightpanel"><div>hola</div></yield>
</my-splitter>

my-splitter.rt:

<tree>
   <yield from="leftpanel"/>
</tree>
<panel>
   <yield from="rightpanel"/>
</panel>