Include only certain line numbers about pandoc-include HOT 4 OPEN

I had no plans for this feature, but it sound useful, and it should be feasible. Please give me some time to think about how this should be done!

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@steindani Thanks so much for considering the idea! I'd still find it useful and would be happy to test it if you decide to give it a shot.

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@joewiz what do you think about the following include syntax?

```cropped
alpha.md <- the source file
2 <- number of skipped lines (0 to start from the first line, optional)
1 <- number of lines to include (optional)
```

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@steindani I'm far from a pandoc or markdown extension syntax expert, but this looks great! My only thought is to change "cropped" to "include-lines". Cropping might suggest image slicing, whereas include-lines seems like a natural extension of your original function. Just an idea though!

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