Added a builtin "nested_tags" pre-processor

This very special and quite simple pre-processor adds support for nested tags (e.g. `{link url={route {config "route_name"}}}`) to Dwoo.

It worked really great without any problems over the last years. Not entirely sure how it behaves when it finds delimiters in string parameters (e.g. `{link text="My awkward {link} text"}`) though.

I leave it open to you whether this should be part of the official distribution. I contribute it as it's a quite small and optional addition but has a great effect on the syntax.