Implement template inheritance via `{{% BLOCKS }}` pragma.

This is a proof-of-concept implementation of template inheritance via blocks.

Say we have a template named `layout.mustache`. This would be our generic site layout template, consisting of all the sorts of things you'd expect in a layout template. In addition, it would have a number of blocks defined. The blocks look like Mustache sections, but they're denoted with `{{$ block }}` tags instead of `#`.

```html+jinja
{{% BLOCKS }}
<!-- layout.mustache -->
<!DOCTYPE html>
<html>
  <head>
    <title>{{$ title }}My Alsome Site{{/ title }}</title>
    {{$ stylesheets }}
      <link rel="stylesheet" href="/assets/css/default.css">
    {{/ stylesheets }}
  </head>
  <body>
    <header>
      {{$ header }}
        <h1>Welcome to My Alsome Site</h1>
      {{/ header }}
    </header>
    <section id="content">
      {{$ content }}
        <p>Hello, World!</p>
      {{/ content }}
    </section>
    {{$ scripts }}
      <script src="/assets/js/default.js"></script>
    {{/ scripts }}
  </body>
</html>
```

The blocks defined here are `{{$ title }}`, `{{$ stylesheets }}`, `{{$ header }}`, `{{$ content }}` and `{{$ scripts }}`.

By default, blocks merely delineate sections of the template which *can* be extended (replaced) in subtemplates. If the parent template itself is rendered, the block tags are simply rendered as if they are truthy sections.

This `layout` template is then be extended by other sub-templates, which declare their intention by adding a `parent=layout` to the `{{% BLOCKS }}` pragma tag: 

```html+jinja
<!-- page.mustache -->
{{% BLOCKS parent=layout}}

{{$ title }}My Alsome Page{{/ title }}

{{$ stylesheets }}
  <link rel="stylesheet" href="/assets/css/page.css">
{{/ stylesheets }}

{{$ header }}
  <h1>My page has some items!</h1>
{{/ header }}

{{$ content }}
  <ul>
    {{# items }}
      <li><a href="{{ link }}" title="{{ name }}">{{ name }}</a></li>
    {{/ items }}
  </ul>
{{/ content }}
```

Upon rendering this `page.mustache`, the subtemplate blocks will be substituted for the parent blocks, resulting in (approximately) this:

```html+jinja
<!DOCTYPE html>
<html>
  <head>
    <title>My Alsome Page</title>
    <link rel="stylesheet" href="/assets/css/page.css">
  </head>
  <body>
    <header>
      <h1>My page has some items!</h1>
    </header>
    <section id="content">
      <ul>
        {{# items }}
          <li><a href="{{ link }}" title="{{ name }}">{{ name }}</a></li>
        {{/ items }}
      </ul>
    </section>
    <script src="/assets/js/default.js"></script>
  </body>
</html>
```

In our example, we didn't override every block: the `{{$ scripts }}` block retained its default value. We could also create a new subtemplate which extends `page.mustache`, say `custom-page.mustache`. That subtemplate might only replace the `{{$ content }}` block of `page.mustache`, and inherit the rest.

Note that `page.mustache` could even define new blocks. Maybe add a `{{$ main }}` and `{{$ complimentary }}` section inside `{{$ content }}`:

```html+jinja
{{$ content }}
  <div id="main" role="main">
    {{$ main }}{{/ main }}
  </div>
  <div id="complimentary" role="complimentary">
    {{$ complimentary }}{{/ complimentary }}
  </div>
{{/ content }}
```

Then sub-subtemplates could choose to override just the `{{$ main }}` block, while leaving `{{$ content }}` and `{{$ complimentary }}` intact.