Add 'hidetitle' metadata to suppress printing a title as heading

Issue #584

This PR provides the boolean metadata 'hidetitle' to suppress printing a title as heading.
Goal is to spare a 'notitle' [template][1] (`.. template: notitle.tmpl`) for simple cases.

The hidetitle metadata works on pages and posts.

[1]: https://github.com/getnikola/nikola-site/blob/master/templates/notitle.tmpl