allow override page url and save_as values directly from the metadata

this is similar to the template override implemented in #404
add functional test for save_as/url override

and update functional tests output
add docs for save_as/url override from metadata feature
Merge pull request #623 from bbinet/url-save_as-override

Override page 'url' and 'save_as' directly from the page metadata. Fixes #400.