This 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.

MooseX::Getopt knows about this abstract role, and will use it if available to
load attributes from the file specified by the commandline flag --configfile
during its normal new_with_options.