Allow the PHP formatter to return a renderable array

It would be nice if I could think of the eval'd PHP for the custom formatter as analogous to the code I would otherwise implement in hook_field_formatter_view(). However, the hook returns a render array, whereas you're supposed to return an output string in the custom formatter interface. It would be nice if custom_formatters_engine_php_render() could process either a returned string or a render array.