show unique twig features

[EvanLovely]

This shows off some unique Twig features that really show it's power over Mustache: Template Inheritance using extends and blocks really help out the Templates, and a macro helps with forms in this pull request.

[TxHawks]

It may be worth also including an example of embed. To me, this is where Twig really shines compared with Mustache.

There isn't currently a pattern that is complex enough for showing the power of embed, but it can be used for extending individual UI patterns within pages.

So, as a simplified example, say we use the card UI pattern across several components that share a similar html "shell", but vary in their internal html, we could do:

{#- card.twig -#}
<article class=card {{ extra_classes }}>
  {% block card_body -%}
  {%- endblock %}

  <footer class=card__footer>
    {% block card_footer -%}
    {%- endblock %}
  </footer>
</article>

and then:

{#- some-organism.twig -#}
{% embed "card.twig" with {extra_classes: 'card--b  some-component' %}
  {% block card_body -%}
    include('molecule-a')
    include('molecule-b')
  {%- endblock %}
  {% block card_footer -%}
    include('molecule-c')
  {%- endblock %} 
{% endembed %}

Of course, this is a very simple example, but with more complex UI patterns, this can greatly improve DRYness

[dmolsen]

@EvanLovely -

I don't think we need a pure 1-to-1 with the Mustache demo if there is something special like this that can't really be done in that template language. Feel free to include if you agree.

[EvanLovely]

I think this would be awesome to include as I can totally see the use case! I've made an issue to track it: #6. @TxHawks - hit it up if you want! I might get to it after a bit, but my initial goal was to get this repo further than it's original minimal state and then to show off a couple of the features that demonstrates why Twig could work better than Mustache for some people. Thanks for the idea!!