Add a billing module to the CLI

[zacparker41]

As a user, I should be able to add, update, and remove a credit card on my account from within the CLI so that I can pay for services, such as Tunnelz (pgrok).

prvd billing should prompt the user to

• add a card, if there is no card on file
  -- needs to capture the name on the card, card number, expiration date, security number and billing zip code
• update the existing card
  -- last four of existing card should be displayed along with the expiration date
• remove the existing card

If no card is on file, the user's access is set at the default access level