API for animateCode which uses comments in the code

[HugoGranstrom]

@pietroppeter had the idea for animateCode, that instead of specifying the lines of code to highlight using numbers (eg. 1..3, 4, 5..7), the lines in the code are marked using comments instead. This would remove the need for updating all the line numbers if you change the code. The general idea is this:

animateCode:
  echo 1 # 1 $$$
  echo 2
  echo 1 # 1 $$$
  echo 2 # 2 $$$$

The number specifies the order of highlighting and the $$$ (or something else), is a marker for our parser to find the lines. A number of questions are still unanswered, like what symbols to use and how to implement it in a simple way.