TODO: Persist keypair to disk

[mycognosist]

Keypairs are currently in-memory only. We need a solution for storing and retrieving keypairs from disk.

[cblgh]

what i'm doing in cable-client: storing the keypair in json with the json keys "publicKey" and "secretKey" with hex-encoded values

example from a test keypair in keypair.json:

{
"publicKey":"2a0d488ef1bc4943435c41de7b9305579459b41f00d89be053038a5a030a3b71",
"secretKey":"20c0eaf4195e03f229b3ec9ef6ad010c560b9a94af26ed7e2cd7b957aeff95dc2a0d488ef1bc4943435c41de7b9305579459b41f00d89be053038a5a030a3b71"
}

[mycognosist]

Ah neat, that's nice and easy. This could be a great first-issue for a new contributor :)