Basic auth support

[lauriannala]

• New options to config:

# Example using basic auth.
config :logger, :loki_logger,
  basic_auth_user: System.get_env("LOKI_USER"),
  basic_auth_password: System.get_env("LOKI_PASSWORD")

• Basic auth options are passed to hackney on http request:

[hackney: [basic_auth: {user, password}]]