+ *
+ * ++ Environment variable override ++
+ *
+ * each key has an associated environment variable name. At runtime (i.e. when
+ * "get" requests are performed) a variable with this name will be looked for,
+ * if it's defined it will override the value present (or absent) in the
+ * registry.
+ * Environment variables are _not_ considered when saving the configuration to
+ * a configuration file (via "save_to" function below) .
+ *
+ * Mapping between keys and environment variables is as follows:
+ * - the whole key is uppercased
+ * - each "." is converted to "__"
+ * E.g.: my.foo_ish.application -> MY__FOO_ISH__APPLICATION