Definition of `label` unintentionally includes empty fragments

[esoterra]

I believe that because fragment -> word -> "" is valid, strange labels like a--b and a------------- are technically valid.

label             ::= <first-fragment> ( '-' <fragment> )*
first-fragment    ::= [a-z] <word>
                    | [A-Z] <acronym>
fragment          ::= <word>
                    | <acronym>
word              ::= [0-9a-z]*
acronym           ::= [0-9A-Z]*

I believe something like this would be more correct.

label             ::= <first-fragment> ( '-' <fragment> )*
first-fragment    ::= <first-word>
                    | <first-acronym>
first-word        ::= [a-z] [0-9a-z]*
first-acronym     ::= [A-Z] [0-9A-Z]*
fragment          ::= <word>
                    | <acronym>
word              ::= [0-9a-z]+
acronym           ::= [0-9A-Z]+

There are many other ways we could write it too, but this seems like it would be fairly consistent with the rest of the grammar.

[lukewagner]

Whoops, good catch! I think this is a recent bug introduced by the grammar refactoring in #592. Will write up a PR in a bit, thanks!