This commit adds a `level` option to the `rubric` directive, which propagates a `level` attribute to the `rubric` node,
and allows renderers to select a specific heading level.
Logic for this attribute has been added to the HTML5 and LaTeX builder.
- Remove now-removed configuration values
- Use obvious 'unset' defaults for ``project`` and ``author``
- Prefer 'e.g.' to 'ex.'
- Allow non-list sequence types in various configuration values
- Add types and defaults to every confval directive
