Take OAuth tokens fron global provider options

[mildred]

The main issue is ueberauth/ueberauth#106 and the use case is to allow dynamic ueberauth providers. To make it simple, the OAuth client id and secret are not available through configuration or environment variable, but are dynamic in database.

The GitHub provider needs to be able to fetch the client id and secret from a dynamic source, not only the configuration. I suggest to allow them be put in options(conn).

The code already has some part allowing that. Ueberauth.Strategy.Github.OAuth.client() already takes an options argument that it merges with the global configuration. Only it's not wired in callers.

This is not the only place but for example at https://github.com/ueberauth/ueberauth_github/blob/master/lib/ueberauth/strategy/github.ex#L112 we could do:

  def handle_callback!(%Plug.Conn{params: %{"code" => code}} = conn) do
    module = option(conn, :oauth2_module)
-    token = apply(module, :get_token!, [[code: code]])
+    token = apply(module, :get_token!, [[code: code], options(conn)])