Export / import swarm nodes

[cblgh]

Hyperswarm just landed the ability to export/import bootstrapping nodes via

dht.getNodes, dht.addNodes

in the near-ish future, it would be cool if we could persist & load discovered nodes, such that if the default set of nodes for some reason doesn't work, it allows people to either cycle through discovered nodes, or setup their own nodes and then add them to cabal in some kind of config or similar