Allow extensions to replace template code using BLOCK

Currently there is no way to replace template code in order to achieve what MODX files did with the replace-with and inline-replace-with commands. So it is impossible, to say, add a new class to an existing line of template code, or simply replace an entire chunk of template code with a new chunk.

The template engine supports TWIGs block tags, using

<!-- BLOCK foobar --><!-- ENDBLOCK -->

which in theory would allow any template code wrapped in BLOCK tags in an extension to overwrite BLOCK tags of the same name in the core.

2 problems:
1. There are no BLOCK tags in the core
2. It doesn't work: TWIGs block syntax only works in template inheritance, so it only works when a template is created using

{% extends base... %}

So phpBB needs a means to allow template code replacement (not just injection)