handling for the given expression.
usage is like:  ${data | encoding('utf-8', errors='strict')}

implemented "args" for <%page> tag.  still has a default
"**_extra_pageargs" catchall for now...

- more docs

added filename-relative lookup to TemplateLookup

more pythonlike about scope:  variable assignment now follows python conventions (i.e., you lose access to the enclosing scope version when you assign locally)