[Fix #496] Highlight [[var]] style comments

[rpkarlsson]

Highlight [[var]] style comments as suggested in #496 using the same rules as backtick-code highlighting.

• The commits are consistent with our contribution guidelines.
• You've added tests (if possible) to cover your change(s). Bugfix, indentation, and font-lock tests are extremely important!
• You've run M-x checkdoc and fixed any warnings in the code you've written.
• You've updated the changelog (if adding/changing user-visible functionality).
• You've updated the readme (if adding/changing user-visible functionality).

[bbatsov]

Looks good to me, but you'll need to add also a couple of unit tests and a changelog entry.

[bbatsov]

Thanks!