Support anchored/reference links

[zkamvar]

Summary

This will allow anchored links in the form of [text][rel-link] and
[rel-link]: <URL> "Title" to be processed with the folowing caveats:

1. The URL is assumed to have no spaces
2. The anchor will sink to the bottom of the document (until we find a
   good process for re-introducing text nodes into the document).

This will fix #45

Example

Without anchor link processing

library("tinkr")
f <- system.file("extdata", "link-test.md", package = "tinkr")
m <- yarn$new(f, anchor_links = FALSE)
m$tail(30) # anchor links unprocesed
#> ---
#> title: this tests links
#> ---
#> 
#> ## These are some links that are valid in basic markdown
#> 
#> This is some text [that contains links](https://example.com/1) which
#> [can be `inline`](https://example.com/2) or [can be spread across multiple lines
#> because the link text is JUST TOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO
#> LONG, y'all](https://example.com/3).
#> 
#> Mainly, we want to see if [link text
#> by reference](https://example.com/4) and if links [can be referenced multiple times](https://example.com/1)
#> 
#> This should also [include non-reference links](https://example.com/5)
#> 
#> If you write \[some link text\]\[link2\], that will appear as [some link text](https://example.com/2 "link with title!")
#> 
#> ## This is some extended markdown content {#extended .callout}
#> 
#> This should also include references that use [standalone](https://example.com/standalone) links and
#> footnotes should not be properly parsed and will be considered 'asis' nodes,
#> at least that's what I *believe*\[^footy\]. Maybe this might not pan out \[^but who
#> knows? footnotes are **WEIRD**, man\].
#> 
#> <!-- links go here! -->
#> 
#> \[^footy\]: this is a footnote that
#> should be preserved

With Anchor link processing

m <- yarn$new(f)
m$tail(30) # anchor links now exist!
#> [can be `inline`][link2] or [can be spread across multiple lines
#> because the link text is JUST TOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO
#> LONG, y'all][link3].
#> 
#> Mainly, we want to see if [link text
#> by reference][link4] and if links [can be referenced multiple times][this fun link1]
#> 
#> This should also [include non-reference links](https://example.com/5)
#> 
#> If you write \[some link text\]\[link2\], that will appear as [some link text][link2]
#> 
#> ## This is some extended markdown content {#extended .callout}
#> 
#> This should also include references that use [standalone][standalone] links and
#> footnotes should not be properly parsed and will be considered 'asis' nodes,
#> at least that's what I *believe*\[^footy\]. Maybe this might not pan out \[^but who
#> knows? footnotes are **WEIRD**, man\].
#> 
#> <!-- links go here! -->
#> 
#> \[^footy\]: this is a footnote that
#> should be preserved
#> 
#> [this fun link1]: https://example.com/1
#> [link2]: https://example.com/2 "link with title!"
#> [link3]: https://example.com/3
#> [link4]: https://example.com/4
#> [standalone]: https://example.com/standalone

^{Created on 2021-05-24 by the reprex package (v2.0.0)}

[maelle]

Brilliant as usual 🌟

[maelle]

🙂 🦶

[zkamvar]

After a few false starts, I have run this against the suite of Software Carpentry lessons without error and I think I should be able to merge this now!