major rewrite of application server & frontend

- adds TOML based configuation files
  - with example configuratios in config-examples
- reimplments arcane CLI argument parser as automaton
- adds a new CLI focused arround configuration files
- moves all file utility stuff from `main.rs` to `util.rs`
- moves all AppServer stuff to dedicated `app_server.rs`
- add mio for multi-listen-socket support (should fix #27)
- consistency: rename private to secret

config-examples/.gitignore

@@ -0,0 +1,2 @@
+peer-*-*-key
+peer-*-out

config-examples/peer-a-config.toml

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+public_key = "peer-a-public-key"
+secret_key = "peer-a-secret-key"
+listen = ["[::]:10001"]
+verbosity = "Quiet"
+
+[[peers]]
+public_key = "peer-b-public-key"
+endpoint = "localhost:10002"
+key_out = "peer-a-rp-out-key"
+# exchange_command = [
+#     "wg",
+#     "set",
+#     "wg0",
+#     "peer",
+#     "<PEER_ID>",
+#     "preshared-key",
+#     "/dev/stdin",
+# ]

config-examples/peer-b-config.toml

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+public_key = "peer-b-public-key"
+secret_key = "peer-b-secret-key"
+listen = ["[::]:10002"]
+verbosity = "Quiet"
+
+[[peers]]
+public_key = "peer-a-public-key"
+endpoint = "localhost:10001"
+key_out = "peer-b-rp-out-key"
+# exchange_command = [
+#     "wg",
+#     "set",
+#     "wg0",
+#     "peer",
+#     "<PEER_ID>",
+#     "preshared-key",
+#     "/dev/stdin",
+# ]