Show outages on the network map

[Svedrin]

child of #52

When a target goes down, have the network map show exactly where the outage occurred by drawing all the nodes red that don't respond anymore.

For this to work, we need to store a separate copy of the traceroute while the target is still alive ("last known good traceroute", so to speak), and then render all the nodes red that are missing from the current one when the target is down.

Necessary PlantUML syntax:

<style>
.nodeDown {
    BackgroundColor Crimson
    FontColor White
}
</style>

hide <<nodeDown>> stereotype

node "meshpingpi3" <<nodeDown>> as SELF

Result: