[Feature] Support kramdown footnote syntax

[tadamcz]

From the kramdown docs:

The footnote definition can contain any block-level element, all lines following a footnote definition indented with four spaces or one tab belong to the definition:

This is a text with a footnote[^2]. 

[^2]:
     And here is the definition.
      > With a quote!

This is not displayed correctly in Typora:

[vassudanagunta]

@abnerlee Please rename this issue to "Support multi-block footnotes like GFM, Pandoc and others"

This syntax is also supported by GFM, GitHub, Pandoc, PHP-Markdown, Multimarkdown, Maruku, MarkDig. You can see how the following is rendered by them here.

In essence, the footnote parser should act similarly to the list item parser, except with a minimum indentation of 4 spaces. Doing so would also fix #6206.

support multi-block, structured footnotes[^m]

[^m]: This is a long foot note
      that spans multiple lines in Markdown

      and has multiple paragraphs

      - and even a list. We could add a table, but
        you get the point.

I think it should be a higher priority to fix.