bin/site: add img_filter

A template filter to replace img(number) by the content of template
img.$format. The number is available as variable 'img' inside the
img.$format template.

Can be used in template files with something like :

 [% FILTER $img_f %]
   some text
 [% END %]
p.b.e/tmpl/post.html: filter some html in image name