Improve configuration with config file

This adds explanations to the example config file, renames the file
in order to enable yaml syntax highlighting and fixes the parsing of
the flag specifying where to find the configuration file.

configuration.example.yaml

@@ -0,0 +1,30 @@
+# Configuration of the Poseidon webserver
+server:
+  # Address on which the webserver listens
+  address: 127.0.0.1
+  # Port on which the webserver listens
+  port: 3000
+  # If set, this token is required in the X-Poseidon-Token header for each route except /health
+  token: SECRET
+  # If set, the API uses TLS for all incoming connections
+  tls: true
+  # The path to the certificate file used for TLS
+  certfile: ./poseidon.crt
+  # The path to the key file used for TLS
+  keyfile: ./poseidon.key
+
+# Configuration of the used Nomad cluster
+nomad:
+  # IP address / domain of the Nomad server
+  address: 127.0.0.1
+  # Port of the Nomad server
+  port: 4646
+  # Authenticate requests to the Nomad server with this token
+  token: SECRET
+  # Specifies whether to use TLS when communicating with the Nomad server
+  tls: false
+
+# Configuration of the logger
+logger:
+  # Log level that is used after reading the config (INFO until then)
+  level: DEBUG