7.0.0: Code blocks compliance

• Implemented xp-framework/rfc#341: Drop support for XP 9 and lower
  (@thekid)
• Merged PR #14: Spec compliance testing. Not run in the test suite yet!
  (@thekid)
• Heads up: No longer use the lang attribute on <code>. Instead,
  use class="language-..." in compliance with spec.
  (@thekid)
• Merged PR #17: Emit code blocks as <code> wrapped in <pre>. Solves
  problem described in issue #16.
  (@thekid)