Add support for // highlight-start and // highlight-end blocks

[dweiss]

With larger highlight blocks the highlight-range is quite inconvenient, especially with follow-up changes or automated reformatting. I think a pair of highlight-start ... highlight-end markers would be more convenient in many cases. I was thinking about adding highlight-start{name} but I don't see the point of naming those ranges since they cannot (must not) overlap anyway.