Extract some code to a method for ease of customizing Twig_TokenParser_B...

...lock

Right now I have to copy-paste [the whole class](https://github.com/rybakit/twig-extensions-deferred/blob/ab4fcb841ae1ff3470034ab0131d5adc73c680ba/src/DeferredTokenParser.php) to be able to use an extra key along with the block name ( `{% block foo deferred %}`).