[NetworkGraph] add 'link: type'

[bittorf]

e.g. wired / wireless / tunnel or even more special:
'10baseT-HD' or
'802.11a/n' or
'bluetooth' or
'zigbee' or
'tuntap'

[nemesifier]

It is going to be very hard to standardize this attribute because everybody will want to have a new type added.

I advise to implement this using the properties object, we can discuss again in the future when we will have more implementations.