Source highlighting (two forms)

- Embedded via CodeRay
- Client-side via highlight.js

Both are optional, activated by the source-highlighter attribute

- use coderay for embedded source highlighting
- load coderay lazily
- add coderay as development dependency (for tests)
- use highlight.js for javascript source highlighting
- tests
- set asciidoctor attribute for creating asciidoctor-specific blocks

Right now highlight.js is being loaded by default from a CDN. By setting the highlightjsdir attribute, the user can change this to a relative reference (all paths handled by browser).

There is definitely just a first step. There's certainly more that can be done with this feature. Let's see what the users have to say.