abstract Mouse role for setting attributes from a configfile

 MouseX::ConfigFromFile is an abstract role which provides an alternate
 constructor for creating objects using parameters passed in from a
 configuration file. The actual implementation of reading the configuration
 file is left to concrete subroles.
 .
 It declares an attribute configfile and a class method new_with_config, and
 requires that concrete roles derived from it implement the class method
 get_config_from_file.
 .
 Attributes specified directly as arguments to new_with_config supercede those
 in the configfile.