markdown: support link reference

[gemmaro]

This pull request marks link reference definitions in the markdown document as no-wrap, which avoids unintentional line breaks at the colon in the long link reference definition line.

Here are some limitations below, but I suspect it works in most cases:

• Multiline link reference definition is not supported (Example 193)
• It misinterprets a link reference definition if its link text is invalid (Example 197)
• Link reference definition may interrupt normal paragraph (Example 213)

[mquinson]

Hello @gemmaro, thanks for this PR. I see that it's marked as a draft, so I'll refrain from merging it, but it sounds pretty good to me. I'd love a little note in the NEWS file also, but I can do that myself afterward if you prefer.

What do you plan to do about it before unmarking it 'draft'?

Thanks again,

[gemmaro]

Hello! Thank you for your reply.

I marked it as a draft, since I wrote this late last night and was worried about doing something careless. I would like to try to make an addition to the NEWS file myself (and also install tidyall and see if it passes).

Best,

[gemmaro]

I updated regexp pattern, to accept indentation for example, according to the CommonMark specification. I also applied tidyall to format the code, and added about the change to the NEWS file.

[mquinson]

Merged, thanks.