v0.2.1

Added

• Magpie.Auth.TokenServer no longer requires a refresh token at startup.
  A server started without one sits in an unconfigured state — calls
  return a pattern-matchable
  {:error, %Magpie.Error{summary: "no_refresh_token"}} without touching
  the network — and the new set_refresh_token/3 configures it (or
  replaces the token, for re-authorization) at any time, discarding any
  cached access token unless a valid access_token/expires_at pair is
  seeded. Fresh installs whose token arrives through the OAuth callback now
  work from a plain static supervision tree
• Magpie.Auth.authorize_url/2 accepts extra_params: (keyword list or
  map) for additional Dropbox authorization params such as
  force_reapprove, locale, require_role and disable_signup.
  Params the function already sets cannot be overridden — collisions raise
  ArgumentError