Option to have case insensitive literals in parsers built from EBNF

Hi Mark,

Thanks for this great library. Here's tiny, pretty self-explanatory change I needed, which others may find useful for parsing i.e. SQL-like or pascal-like languages. It is turned on by :string-ci true (or any truthy val) option on core/parser, or via dynamic var cfg/*case-insensitive-literals*.

I've tried not to break any function signatures, hence dynamic var.