Allow outputting the reference number in groups of five digits

This patch adds an option to group the reference number like so: 111 12222 33336. It's easier to read this way. And it's totally legal too: [pdf](http://www.fkl.fi/teemasivut/sepa/tekninen_dokumentaatio/Dokumentit/kotimaisen_viitteen_rakenneohje.pdf)

Also, fixes a typo in README so google can find this library.