Added bin/map-staging-usernames-to-production.js.

Running this script without arguments will print the following help:

  usage: map-staging-usernames-to-production.js <username> <password>

  This script will output a JSON array of makes, with "email" and
  "productionUsername" fields set (though the latter may be null).

  <username> and <password> are the credentials to the production
  login API at https://login.webmaker.org.

  Also ensure that LOGINAPI_URL and LOGINAPI_AUTH point to a
  staging or development instance of the login API (see README.md
  for more information).