Template Language for a CMS

How would you go about creating a template language that could be used
to allow access to *some* helpers for a CMS-style app?

For example if there is a text field called content, I can use Textile
or (my preference) Markdown to allow easy html generation, but they
don't allow for any conditional markup based on the page. Is there any
way to extend these languages by allowing access to some of the helper
methods.

I was thinking something like:

% image(bart.png) %

% breadcrumbs %

% nav_menu(child_pages) %

Does something like this exist? Would it be better to try to extend
markdown or write one from scratch? How would you go about writing
one?

cheers,

DAZ

Thanks Macario,

Liquid looks like what I'm after - and the railscast (as always) is
excellent. It seems like I could hook some specific helper methods up
to it and they could be outputted using {{ this notation }}

Out of interest ... how does one go about writing their own template
language? Any basic way of starting?

cheers,

DAZ