Do not cascade configuration files

Currently, it is possible to have several layers of user configuration files. These are the configuration file in the home directory and those given by the `BEETSDIR` environment variable and the `--config` option. This means that when giving a configuration file on the command line the options from the configuration in the home directory are also set. In my opinion this behaviour is confusing and intransparent.

Is there are reason for this and is anyone relying on it?