print the path to the configuration file used

when verbosity > 0, it prints the path to the configuration file used
the only problem with this is that if your config explodes in config_loader#load, merge_configs or apply_computed_config then you won't get informed of what config you were using.

you might want to rescue exception around those functions, then print which config file error'd (using gold $stderr.puts, not the mis/un-configured logger) and then re-raise the exception.