added support for traits (experimental)

added support for traits (experimental)

Traits are "special" templates that define blocks that you can "include" in
other templates.

Let's take an example:

    {# 'foo' template defines the 'foo' block #}
    {% block 'foo' %}
        FOO
    {% endblock %}

    {# 'main' template defines the 'bar' block and include the 'foo' block from the 'foo' template #}
    {% extends 'base' %}

    {% use 'foo' %}

    {% block 'bar' %}
        BAR
    {% endblock %}

In the previous example, the 'main' template use the 'foo' one. It means that
the 'foo' block defined in the 'foo' template is available as if it were
defined in the 'main' template.

You can use as many 'use' statements as you want in a template:

    {% extends 'base' %}

    {% use 'foo' %}
    {% use 'bar' %}
    {% use 'foobar' %}

If two templates define the same block, the latest one wins. The template can
also overrides any block.

The 'use' tag also supports "dynamic" names:

    {% set foo = 'foo' %}

    {% use foo %}

The 'use' tag only imports a template if it does not extend another template,
if it does not define macros, and if the body is empty. But it can 'use' other
templates:

    {# 'foo' template #}
    {% block 'foo' %}
        FOO
    {% endblock %}

    {# 'bar' template #}
    {% use 'foo' %}

    {% block 'bar' %}
        BAR
    {% endblock %}

    {# 'main' template #}
    {% extends 'base' %}

    {% use 'bar' %}

In this example, the 'main' template has access to both 'foo' and 'bar'.

Traits are mainly useful when you consider blocks as reusable "functions";
like we do in Symfony2 forms or if you use Twig for code generation.