dtmapi 0.1.0

To manage this requirement in the dtmapi package, a new function, set_subscription_key(), is introduced. Calling it without arguments prompts the user to enter their subscription key, which will make the key available for a given R session through the environment variable DTM_SUBSCRIPTION_KEY.

The functions which make API calls (such as get_idp_admin1_data()) have been changed to make the subscription key that is assigned to the environment variable part of the API request, since a valid key is a requirement for the API.