All role specific proxy configuration

By default only the primary role runs the proxy. To disable the proxy
for that role, you can set `proxy: false` under it.

For other roles they default to not running the proxy, but you can
enable it by setting `proxy: true` for the role, or alternatively
setting a proxy configuration.

The proxy configuration will be merged into the root proxy configuration.

lib/kamal/commands/app.rb

@@ -1,5 +1,5 @@
 class Kamal::Commands::App < Kamal::Commands::Base
-  include Assets, Containers, Execution, Images, Logging
+  include Assets, Containers, Execution, Images, Logging, Proxy
 
   ACTIVE_DOCKER_STATUSES = [ :running, :restarting ]