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become outdated if a new version of acoc is installed.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.
Merge pull request #4 from benoit-pierre/config-handling

Change config handling