Support line breaks and and lazy continuation in footnotes

[vassudanagunta]

GFM, GitHub, Pandoc, PHP-Markdown, Multimarkdown, Maruku, MarkDig all support lazy continuation line breaks in footnotes. Typora does not.

See here.

support line breaks and lazy continuation in footnotes[^m]

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

I think this should be fixed in combination with #4569. In essence, the footnote parser should act similarly to the list item parser, except with a minimum indentation of 4 spaces.