Add support for zones

[cmahnke]

I'm currently looking d3-tube-map for process visualisation, it would be nice to have a possibility to add some context. Translated to a Tube map these would be the travel zones - for London those would be 1 till 6 depending on the distance from the center. I guess that it would be way out of scope to calculate these for this project, so I propose adding just a number to each station section and an additional zones section on the same level as stations which would be used to provide a display name for each zone.

Great project btw!