Add HTML-like text control elements

[quinchs]

Summary

It would be a nice-to-have if we allowed different text control elements like <h1>, <br/>, <b> etc to be rendered as markdown in the final builders. Implementation for this may require some more abstraction to property renderers since as it stands the property renderers work with a single CXValue which doesn't represent text and elements.

Sample

<text>
    This would be <b>bold!</b>
</text>

Generated:

"This would be **bold!**"

For context, the text element uses property substitution to allow child cx nodes to be interpreted and rendered using the string renderer. Further logic is needed to interpret and correctly parse more than 1 child.