Display reference links, in footer / where specified, global (or incremental)

**Is your feature request related to a problem? Please describe.**

I would like a way to display the label and link of a reference-link, either all-together via a `[REFS]` macro, or at the point of definition indicated by, if needed, some other syntax ( :/ )
The reference links allow

Reference[rs]: `[text][label]` or `[text label]`; and the associated reference[es]: `[label]: https://linky/link`

**Describe the solution you'd like**

Analogously to the table of ~context~ contents, `[TOC]` link, a `[REFS]` shortcut would be appreciated to have the accumulated links displayed in a group, as defined, with ref label and link spelled out fully.

Or an option of displaying them where you choose to define them, whether in a section footer or document footer, or just near first usage. `~[]:

I could undertake a more serious consideration of what syntax would be least intrusive. Maybe something inside the label portion of the link which could be used to mean display-intent, or escaped for literal inclusion.

**Describe alternatives you've considered**

I've manually included them myself, which I would not like to repeat

```
A [comic][xkcd] on the web, and a [site for sharing code][github]

Manual listing without spelling out link:
- [xkcd][xkcd]
- [github][github]

[xkcd]: https://m.xkcd.com/
[github]: https://github.com/
```

**Additional context**

I would like this feature considered because copying the document into another context often does not allow the keeping of hyperlink metadata, and this feature, with full link spell-out, would make HackMD a more attractive / useful place to draft documents which I may copy, use, and modify in another context.