Improve annotation process

- if pygments is availlable, highlight

- when lines have no generated code, do not generate the subdiv and do not
  make clickable

- line number with correct number digit for alignment of code

- line that can be expanded with + in front

- use 0 margin top and bottom for code blocks
Merge pull request #290 from Carreau/clean-annotate-2

line-highlight fix + pygments highlight