Footnote support for markdown

[fenbox]

Is your feature request related to a problem? Please describe

The current Markdown footnote is not parsed correctly.

Describe the solution you'd like

Parse in a common way.

Example:

A clear and concise description of any alternative solutions or features you've considered.

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.

    `{ my code }`

    Add as many paragraphs as you like.