Extracts config section code into a module
Extracts code to setup new clients

Both commands to add clients had most of the same copy/pasted logic with
minor variations and also had to know the format of the config file /
key names.

This hides that behind a method on the config
Adds check for email argument being present
Set the default client if first added

The first client added to the configuration gets set as the default
client to make getting started easier.