EEPROM atoms: add an optional URL atom

One of the HAT spec goals is to significantly lower the learning curve for add-on hardware usage. Similarly, we can expect HATs with an educational theme to be more than "plug-and-play" and have programmability or firmware components.

HATs can be self-documenting (or at least point to internet resources for manuals, guides etc). Add a new optional Atom data type for nodes in JSON containers.