Reference-style footnote definitions should be flanked by blank lines, above and below

[jcarpenter]

Per Pandoc Footnotes spec:

Each footnote should be separated from surrounding content (including other footnotes) by blank lines

[jcarpenter]

Not sure how we'd implement this in markdown.js. It would require checking the preceding and following lines, which we haven't had to do yet. Probably in blockNormal function?