Consider CRLF when splitting code lines

[notZaki]

I noticed that code lines ending with # hide were not getting hidden in the output.
The problem went away if I switched from CRLF to LF line endings.
This might be related to issue #613

I think the issue is caused by split(code, '\n') (line 680 in expanders.jl) which assumes LF line ending.
For CRLF input, code containing ... # hide gets split to ... # hide\r and this causes the regex on line 681 to not match---this is because $ does not match \r [ref]

This PR fixes that by splitting code with r"\r?\n".

---

Minimum working example:

• make.jl

using Documenter
makedocs(sitename="Example")

• src/index.md (using CRLF)

# Example

```@example
import Random # hide
Random.seed!(1) # hide
A = rand(3, 3)
b = [1, 2, 3]
A \ b
Random.seed!(2); nothing # hide

• Ouput using master (note that last line still gets hidden correctly):
  

• Output using PR
  

• If LF is used, then both master and PR have the same output where code is hidden correctly

[mortenpi]

Great, thanks! LGTM!

Could you also add a bugfix entry to CHANGELOG.md for this, under v0.22.0?

[mortenpi]

Some parallel processing here 😆 Thanks! Will merge when tests go green.

[notZaki]

Updated changelog.
Didn't know if the policy was to squash commits or not, so I forced push the changes to avoid unnecessary commit messages.

[mortenpi]

I squash merge PRs usually, so no need to squash them by hand.