change `builtinId` to `numFmtId` so it works in Excel 2007 and others - hopefully fixes #7

thanks @marcheiligers and @SheetJSDev (who really knows his XLSX)
update README with corp support
update license and copyright
lowerCamelCase seems to be standard
use MurmurHash3::V128.str_digest instead of Digest::MD5 for shared strings
modernize
bump patch