Add BNF specification

[david-a-wheeler]

I'm surprised that a spec like this doesn't include a concise specification using BNF. I think it should be added.

You could use the BNF notation used by XML (and OpenFormula), it's pretty clean:
http://www.w3.org/TR/2006/REC-xml11-20060816/

[Bengt]

The request for a formal specification has come up in talk before, including attempts at an (A)BNF:

http://talk.commonmark.org/t/abstract-syntax-tree-is-more-important-than-implementations/521
http://talk.commonmark.org/t/vanilla-flavored-markdown-as-basis-for-state-machine-spec/362

[jgm]

If anyone wants to contribute a BNF, please do! But I'm very skeptical that it can be done, due to the many quirks of the syntax. Closing this for now.