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
bump 0.3.0
Guard against nil node replacement results
Merge pull request #84 from jbarnette/patch-1

Guard against nil node replacement results
bump version 0.3.1
Update readme with link to asciidoc filter
Merge pull request #87 from jch/jch-patch-1

Update readme with link to asciidoc filter
add dependency on activesupport to access xml_mini package
Merge pull request #85 from mojavelinux/xml-mini

use xml_mini extracted from activesupport
Merge upstream/master; activesupport is a full dependency