Feature: Add new accounts to config file with [add] command

[pbnj]

Users should be able to add accounts to the config file without having to modify the file manually.

Example:

• Interactive mode:

  $ assumer add 
  ? New Control Account [y/N]: N
  ? New Target Account [Y/n]: Y
  ? Target Account Name: new-account-2
  ? Target Account Number: 222222222222