Configuration file loading must be incremental

The loading of the configuration file follows the precedence order (from the high to low priority): local ⇒ user ⇒ system ⇒ package. In the present code, once a configuration file is found, the other ones with lower priority are not considered. However, since the configuration is essentially a dict (in YAML format), files down the precedence list may contain keys that do not appear in files with high priority. The code must be changed in order to cope with this situation and parse all the possible configuration, retaining the keys that appear in the higher priority ones.