Add footnote syntax?

As discussed in #31 it appears that people are for a footnote syntax, as implemented in some flavours of Markdown (see Extended Syntax):

Here's a simple footnote,[^1] and here's a longer one.[^bignote]

[^1]: This is the first footnote.

[^bignote]: Here's one with multiple paragraphs and code.

    Indent paragraphs to include them in the footnote.

Can we come to a decision on this, bearing in mind

1. that this syntax extension is not strictly compliant with CommonMark, as discussed here
2. That (as with any syntax extension) this would incur some parsing performance cost (although probably minimal)

Note these footnotes would be inherently auto-numbered, as can be tried in the VS Code standard preview (GitHub markdown does not support footnotes)

paging @choldgraf @mmcky @jstac @akhmerov @najuzilu

[chrisjsewell]

What are your thoughts on inline footnotes? On the one hand, RMarkdown already supports this,

I’d say no because its pretty non-standard and could just as easily be handled in a simple extension:

{foot}`this is a footnote to be auto-numbered`

rather than introducing more complexity into the parser, leading to a slower parse and more potential bugs.

[choldgraf]

yeah, well it would probably just directly go from [^this syntax] to the docutils node for a footnote (at least, I think there’s a special node for this). If we wanted to add an in-line role for a short-hand, it might be something like {ft}`This is my footnote text`