#214, rt-template with in-place child component

[yatra9]

This implements #214

I add tests, and modified README also (but, review required, because I'm not a native English speaker)

[nippur72]

great, I have two suggestions:

• can we rename in-place with something more intuitive, e.g. component ? Or perhaps we could have a new attribute <rt-prop>:

<rt-template prop="mybutton" component>
   <button>my</button>
</rt-template>

<!-- or -->

<rt-prop name="mybutton">
   <button>my</button>
</rt-prop>

• just for code styling, use string interpolation when throwing errors:

  throw RTCodeError.build(context, child, `'rt-template' should have a '${templateNode_PropAttr}' attribute`);

  instead of the more verbose

  throw RTCodeError.build(context, child, "'rt-template' should have a " + templateNode_PropAttr + ' attribute');