Define the grammar for EBNF-ish spec entries

[wking]

We currently have a digest grammar with no defined grammar syntax. I had previously floated transitioning that grammar to the well-defined ABNF, but #655 was rejected. There has recently been a lot of discussion about various grammar syntax in #671, but no consensus around picking an externally-defined grammar or adding local definition for the grammar. Many of the external grammars are very similar to the current digest grammar:

digest                := algorithm ":" encoded
algorithm             := algorithm-component [algorithm-separator algorithm-component]*
algorithm-component   := /[a-z0-9]+/
algorithm-separator   := /[+._-]/
encoded               := /[a-zA-Z0-9=_-]+/

being very similar to XML 1.0 EBNF:

digest                ::= algorithm ":" encoded
algorithm             ::= algorithm-component (algorithm-separator algorithm-component)*
algorithm-component   ::= [a-z0-9]+
algorithm-separator   ::= [+._-]
encoded               ::= [a-zA-Z0-9=_-]+

After extensive discussion on IRC today (starting here), @stevvooe offered to define the grammar if I filed an issue, so here it is.