Support multiline footnotes

[zegl]

Many services, including GitHub, supports multiline footnotes on following format¹:

[^format]: row
row
row

and an indented format that allows for empty lines inside the note²

[^indented]: row
    more

    footnote after empty line

footnote after empty line

They also support a limited set of markdown in footnotes³:

[^md]: row
**row**
`row`

markdown-to-jsx currently only supports single line footnotes⁴.

[^single]: single line

I'm proposing to add support for multiline footnotes in markdown-to-jsx as well, which I would be willing to contribute the code for.

Footnotes

1. row
   row
   row ↩

2. row
   more ↩

3. row
   row
   row ↩

4. single line ↩