Change config handling

* do not create user configuration if missing

    Rational: automatically creating a user configuration file means it'll
    become outdated if a new version of acoc is installed. It's better to
    let the user decides himself to make a copy if he wants to make some
    custom modifications.

* prepend internal acoc.conf to config files list

    This way acoc can be run from source, or from gem install without the
    need for a configuration file in one of the standard locations.