Support for {% block.super %}

One of the nice features from Django's templating is the ability to add to an existing block (instead of overwriting it). It would be nice if this support was also built into Complexity.

Start by locating Complexity's template parsing and rendering entry points and any related tests; the issue provides no file or test names. Compare the requested behavior with Django's block.super semantics, then define completion as allowing a block to extend existing block content without replacing it, with coverage for that behavior.

Written by the indexing model from the issue text.