Fix #417

- Avoid appearance of dark border around visited code links in Safari.

Optimised CSS selectors.
Changed markup in `linenos-test` to inhibit line numbers appearing in YAML examples when testing code fences.

The additions to `_config.yml` go together with the changes to `code.scss`,
to facilitate adjusttments by users. See `docs/linenos-test` for the details.
The CSS code has been significantly refactored and simplified,
and seems to produce sensible results (at least on Safari and Firefox).

Removed some comments and layout

Produces sensible (but not perfect) results from different kinds of highlight markup.
The CSS code needs a major refactoring to eliminate duplication and superflous lines.

Fixed more layout details. Numbers of selectors still to be fixed.

Correected layout, order, values.

The added examples in `docs/index-test.md` extend the previous examplees of highlighting, documenting the required inout.