Teach SyntaxHighlightFilter about a :highlight context attribute

This allows any code blocks without a language specified (indented
blocks or fences with no language) to be highlighted with a default.

For extra fun, add an additional class to the "highlight" div
specifying the highlighted language.
Simplify tests for older test/unit
Merge pull request #81 from jbarnette/default-highlight

Teach SyntaxHighlightFilter about default highlights