Read auth information from env

[mythmon]

This does a few things related to how the authentication is loaded.

• It will read from an envvar OBSERVABLEHQ_AUTH, overriding the config file if the envvar has a non-empty value.
• It annotates where the auth value was loaded from so we can give more useful error messages to users.
• It changes the interface of the auth-getting function to always return an API key. If it can't find a key, it will give the user an error message and exit the program.
  • The test version of the function throws an error instead.
  • In the future we could prompt the user to login with their browser at this point as well.
  • If we ever have a case where we want to make the API key optional we could further extend this function with a required: boolean parameter.