Support for nested extends from different templates

Support for nested extends from different templates. Related ticket #5

It does not have to load templates twice and it works with arbitrary combinations of templates extending each other. It does require use of `DebugLexer` which can be achieved by setting `TEMPLATE_DEBUG` to `True` or by monkey-patching Django.

See related comment here: https://code.djangoproject.com/ticket/17199#comment:9