Add a possibility to use different OTP apps

[ihorkatkov]

There is a case, when different otp apps inside umbrella needs to use the library. The PR adds an option to configure it basing on an OTP app.

Like so:

config :my_app, Ueberauth.Strategy.ADFS.OAuth,
  adfs_url: System.get_env("ADFS_URL"),
  adfs_handler: MyApp.ADFSHandler, # Optional, ability to provide handler to extract information from the token claims
  client_id: System.get_env("ADFS_CLIENT_ID"),
  resource_identifier: System.get_env("RESOURCE_IDENTIFIER")

config :my_app, Ueberauth,
providers: [
  adfs: { Ueberauth.Strategy.ADFS,
    [
      otp_app: :my_app
      #uid_field: :email,
      #request_path: "/auth/adfs",
      #callback_path: "/auth/adfs/callback"
    ]
  }
]

[ihorkatkov]

The reason it has conflicts is that we use 0.2.0 version of the library. If you want to use the functionality, feel free to use the branch and rebase on the master. Thanks