Highlighting code blocks: leading spaces should be counted

[damithc]

Context: I was trying to highlight these leading spaces, but was not able to.

Consider the following code:

```{highlight-lines="[:4]"}
  abcd
````

Expected: highlight ab and the two spaces in front
Actual: highlights abcd

So, it seems the current counting doesn't include leading spaces.

[yucheng11122017]

Hi @damithc I think this was the intended behaviour when this feature was introduced in #1478 by @ryoarmanda. Making this correction to include leading spaces would be a breaking change since users would have to adjust their spacing accordingly. Is the use case for this important?

[damithc]

Hi @damithc I think this was the intended behaviour when this feature was introduced in #1478 by @ryoarmanda. Making this correction to include leading spaces would be a breaking change since users would have to adjust their spacing accordingly. Is the use case for this important?

@yucheng11122017 Ah, I see. I guess we can look for a way to support the requested behaviour in an non-breaking way, or introduce it as a breaking change together with a major release.