added support for parenthesis in if/elseif
Template inheritance.

Content can be captured via the <% content NAME { %>...<% } %> tag and
exported via the built-in yield method. Yield is a real core.Value, and thus can
be used in an assignment or any other place where a core.Value is supported.

Support for the default yield (which dump any content not in an explicit
content tag) is pretty hackish and done at the parser level. This is
unfortunate.
configuration section