Syntax highlighting, mark-as-executed, transitional CSS, rendering refactoring

[hemidactylus]

Four independent improvements put together:

• modified to rely on the transitional "katapodv2.css" to better iterate without damaging previous version
• syntax highlighting implemented through highlight.js (ordinary marker at the three-backtick symbol, accepts "cql")
• rendering code, refactored for better modularity
• mark-as-executed for code blocks (uses messaging between extension and rendered page)