Add message signing

All messages are now signed by default with a randomly generated key.
TODO still is keyserver stuff, locking down the config file potentially
(or telling users to do it in the readme?), and doing any sort of
verification

Cargo.toml

@@ -21,3 +21,5 @@ time = { version = "0.3.47", features = ["serde"] }
 hickory-resolver = "0.26.1"
 expanduser = "1.2.2"
 uuid = { version = "1.23.2", features = ["v4"] }
+ed25519-dalek = { version = "2.2.0", features = ["rand_core", "serde"] }
+rand = "0.8"