Replace args.* tests with parser.add_argument

This replaces the tests for defining an aternate config file and
sending the geojson to geojson.io, by using the parser.add_argument
'dest and 'default' options.
Handle geojsonio.py import exceptions

This will print a message telling where to get the modules you need
if you try to run 'archmap.py' without 'github3.py' or 'geojsonio.py'.