[Installer] Allow disabling default plugins

[Validator] Disable default plugins when installing

lib/cocoapods/installer.rb

@@ -67,6 +67,8 @@ module Pod
       @sandbox  = sandbox
       @podfile  = podfile
       @lockfile = lockfile
+
+      @use_default_plugins = true
     end
 
     # @return [Hash, Boolean, nil] Pods that have been requested to be
@@ -76,6 +78,12 @@ module Pod
     #
     attr_accessor :update
 
+    # @return [Boolean] Whether default plugins should be used during
+    #                   installation. Defaults to true.
+    #
+    attr_accessor :use_default_plugins
+    alias_method :use_default_plugins?, :use_default_plugins
+
     # Installs the Pods.
     #
     # The installation process is mostly linear with a few minor complications
@@ -461,7 +469,11 @@ module Pod
     # @return [Hash<String, Hash>] The plugins to be used
     #
     def plugins
+      if use_default_plugins?
         DEFAULT_PLUGINS.merge(podfile.plugins)
+      else
+        podfile.plugins
+      end
     end
 
     # Prints a warning for any pods that are deprecated

lib/cocoapods/validator.rb

@@ -342,6 +342,7 @@ module Pod
       podfile = podfile_from_spec(consumer.platform_name, deployment_target, use_frameworks)
       sandbox = Sandbox.new(config.sandbox_root)
       installer = Installer.new(sandbox, podfile)
+      installer.use_default_plugins = false
       installer.install!
 
       file_accessors = installer.aggregate_targets.map do |target|