Add a handful of backwards compatible extensions:

[wilbowma]

I really like the package; it's a much better interface than alternatives, and better than my hand-rolled attempts. I found myself adding some features I commonly need in my work, so I thought I'd offer them upstream. Sorry for the macro commit.

Examples and documentation are included: https://github.com/wilbowma/simplebnf/blob/master/simplebnf-doc.pdf

1. Support set membership declarations in the grammar, useful for defining
   infinite values like variables or natural number literals as part of the
   grammar.
2. Add LHS annotations.
3. Add optional argument for specifying tabular alignment.
4. Add support for non-line-breaking RHS productions.
5. Add optional arguments for parsing breaking vs non-breaking productions.

[Zeta611]

These enhancements seem very useful for me as well!
I will upload the updated version to CTAN.

Thank you very much for the contribution.