Add IDs to references to allow crossreference and avoid duplicates

[Sogolumbo]

In the Markdown Syntax it's possible to give id's to links and specify the url somewhere else. I think it would be very good to have the same with footnotes.
It would be easy to implement in such a way that one reference can be cited multiple times and cross references to other citations are possible.

Here's an idea what it could look like:

this is an example labeled reference [refid] ny-times [/refid].

We can cite the same reference again [refid] ny-times [/refid] 
and create a footnote that has a crossreference to the first footnote [refid] cross-ref [refid].

Then, anywhere in the document, you define the reference markdown:

[refcontent] ny-times: New York Times [homepage](https://www.nytimes.com/) [/refcontent]
[refcontent] cross-ref: See headline of [refid] ny-times [refid]. [/refcontent]