Route macros, after_install callback, payment guards

• use simple route macros instead of writing out boilerplate routes and pipelines.
• add after_install callback which can be overridden in AuthController
• BETA payment guards. Use the Shopifex.Plug.PaymentGuard plug. If the merchant does not have an active grant associated with the named guard, it will redirect them to a plan selection page, allow them to pay, and handle the payment callback all automatically.
• a few other conveniences related to config options.