feature request: add support for multiple tor instances

[nusenu]

I'm filing this here as a github issue so I get notifications if there are any updates on this. I mentioned this previously on the tor-relays ML [1].

Rationale:
Many relay operators run multiple tor instances, either on a single host or multiple hosts.
Instead of running multiple onionbox instances, ops would probably appreciate if they would have a single overview over their relays.

Each instance is simple added in a stacked way to the graph.

Maybe onsider two modes:

• online (onionbox gets access to their controlport)
• offline (onionbox only gets their data from onionoo)
  this is usefull for multiple tor instances that are not run on a single host

[1] https://lists.torproject.org/pipermail/tor-relays/2016-September/010409.html