- [feature/bug] Can now refer to context variables

  within extra arguments to <%block>, <%def>, i.e.
  <%block name="foo" cache_key="${somekey}">.
  Filters can also be used in this way, i.e.
  <%def name="foo()" filter="myfilter">
  then template.render(myfilter=some_callable)
  [ticket:180]