dtmapi 0.1.0

• Version 3 of the DTM API requires the use of a valid subscription key in order
  to make requests and retrieve data from it. A subscription key may be obtained
  through the DTM API Portal.

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.

• Introduces get_subscription_key(), which is mostly used to develop and test
  the dtmapi package, but it may technically be used by users to conveniently
  return the currently available subscription key. HOWEVER, beware that the string
  it returns is plain text and is more easily exposed, e.g. exposed to anyone
  visually observing your machine.