MAJOR Introduced '-c, --config' option.

Changes:
    -- introduced new '-c, --config' option that allow to specify a path to the configuration file (by default we still will search this file in current working directory);
    -- reorgonised 'configuration' routine, created platform dependant extensions;
    -- 'is_invalid' is now part of configuration file.
DOC Added description of '-c, --config' option.