Inline Footnote support

[benaryorg]

Problem

Having to split the footnote reference and content into two places makes for several pitfalls including mismatched names, moving one to a different document but not the other, etc..

Proposed Solution

Pandoc has an extension for inline footnotes.
It would be great if mdbook could mirror this feature.

Notes

(sorry if I missed existing issues or similar, I tried to look)