Separate 'source' and 'repository' into separate config sections

Code courtesy of Joshua Rountree (@joshuairl)
Allow using different credentials for the different repositories
Allow specifying the server in the config

This allows for enterprise organization to use the script.

Code courtesy of Joshua Rountree (@joshuairl)

Also cleaned up the prompt for username/password.
Add notice about Enterprise support in README
Add support for 'GitHub Enterprise'