Abbreviation extension can not have text on multiple lines

[NicolasMassart]

To prevent having too long lines, which is checked by markdown linters, that would be nice to be able to write abbreviations on multiple lines. Today it only work on one single line:

*[THTBOOSL]: This has to be on one single line.

[waylan]

I expect that Markdown linters don't know anything about a abbreviation definition. If they aren't complaining about them at all, then that is coincidental (in that the linter isn't strict enough). We should expect that all abbreviation definitions will trip up a linter. Therefore, the fact that long lines trip up the linter is a read herring.

That said, I suppose there might be some argument for allowing long lines to be wrapped (easier to read, etc.). But, how long are your abbreviations anyway? Wow. I don't think I ever considered this to be a problem.

In any event, this is not a high priority item for me. However, I would be willing to review a PR if someone else wanted to do the work.