[Markdown] Reference-style links not working in posts

[fuzzmz]

Reference-style Markdown links aren't working.

In the following example the first markdown link (Google) doesn't get turned into a hyperlink once the post is made, while the second one (this one) works as expected:

This [Google][1] link doesn't work while [this one](https://google.com "Google") does.
    [1]: https://google.com "Google"

Ties to flarum/framework/issues/630

[JoshyPHP]

I replied on the other page but I'll leave a comment here as well. I think that the syntax for reference links is cruft and implementing it would make the library worse. I don't think there's a use case for it in modern applications.

[fuzzmz]

I have nothing against not having this feature. I only came across it as I've been toying with integrating @flarum as a commenting system for a @pelican powered blog, and was using the article summary as forum post contents, which in my case happened to use reference-style links.

[JoshyPHP]

I've thought about it and decided to implement reference links and images. I don't think anyone will really use it but I was able to implement it without adding too much code and without any impact on the runtime performance. The syntax is consistent with the original Markdown, if a little bit more flexible.

For reference, this is the new supported markup: (compare it on Babelmark 2)

* [label]
* [text][label]
* ![label]
* ![text][label]
* [![label]][label]

[label]: http://example.org/img.png "Title"

[fuzzmz]

Thanks @JoshyPHP! 👍