Allow basic [[NewPage]] usage without specifying a parent= parameter

Currently, if you don't specify a parent= parameter, the macro will still render without any apparent error, but the form it renders will contain a `<input type="hidden" name="parent" value="undefined" />` which will cause an error when it's used.

This patch makes parent= optional -- if it's absent, a default value of "/wiki" is assumed.

(And/or: would it make sense to prepend /wiki to any "parent" value that doesn't start with /wiki?  I'm not sure, but it seems like that could be a common error, especially since the "template" parameter is just a page name under `/wiki/PageTemplates` and not a full URL path.)