Add a require_destination setting

If you always want to use a destination, and have a base deploy.yml file
that doesn't specify any hosts, then if you forget to specific the
destination you will get a cryptic error.

Add a "require_destination" setting you can use to avoid this.

lib/kamal/configuration.rb

@@ -9,8 +9,7 @@ class Kamal::Configuration
   delegate :service, :image, :servers, :env, :labels, :registry, :stop_wait_time, :hooks_path, to: :raw_config, allow_nil: true
   delegate :argumentize, :optionize, to: Kamal::Utils
 
-  attr_accessor :destination
-  attr_accessor :raw_config
+  attr_reader :destination, :raw_config
 
   class << self
     def create_from(config_file:, destination: nil, version: nil)
@@ -120,6 +119,10 @@ class Kamal::Configuration
     "#{service}-#{version}"
   end
 
+  def require_destination?
+    raw_config.require_destination
+  end
+
 
   def volume_args
     if raw_config.volumes.present?
@@ -161,7 +164,7 @@ class Kamal::Configuration
   end
 
   def valid?
-    ensure_required_keys_present && ensure_valid_kamal_version
+    ensure_destination_if_required && ensure_required_keys_present && ensure_valid_kamal_version
   end
 
 
@@ -243,6 +246,14 @@ class Kamal::Configuration
       true
     end
 
+    def ensure_destination_if_required
+      if require_destination? && destination.nil?
+        raise ArgumentError, "You must specify a destination"
+      end
+
+      true
+    end
+
 
     def role_names
       raw_config.servers.is_a?(Array) ? [ "web" ] : raw_config.servers.keys.sort