code: public static String setProperty(String key, String value) {
          return INSTANCE.setProperty(key, value);
      }
docs: Set a property in the ConfigFactory. Those properties will be used to expand variables specified in the `@Source`
  annotation, or by the ConfigFactory to configure its own behavior.
  
  @param key   the key for the property.
  @param value the value for the property.
  @return the old value.
  @since 1.0.4