allow the object of the {% createphp %} tag to be any expression

this is strictly backwards compatible. the only difference is that now you can do something like

    {% createphp container.field.content %}

instead of having to use a direct variable present in the template, or a cludge like

    {% set content = createphp container.field.content %}
    {% content %}

At the same time, i make the `=` of `as="myname"`` optional to allow the more natural {% createphp content as "myname" %}