Support relative callback and request paths in strategies

[hugomatinho]

When dynamically changing the base_path callback and request paths are absolute thus disabling the dynamic nature of the call.

I'd like to propose that the strategies' callback_path and request_path be relative to the base_path

for example when using gettext and ueberauth, gettext changes the base path to /:locale even adding to the ueberauth base_path doesn't help as the strategy paths are absolute. defeating in this case the localization intent

config :ueberauth, Ueberauth,
  providers: [
    facebook: { Ueberauth.Strategy.Facebook, [
      default_scope: "email, public_profile, user_friends",
      profile_fields: "name, email, first_name, last_name",
      request_path: "/en/auth/facebook",
      callback_path: "/en/auth/facebook/callback"
      ]
    },
    identity: { Ueberauth.Strategy.Identity, [
      callback_methods: ["POST"],
      request_path: "/en/auth/identity",
      callback_path: "/en/auth/identity/callback"
    ]}
  ],
  json_library: Jason,
  base_path: "/:locale/auth"

current workaround is to duplicate providers per each lang for gettext