Allow Custom PMixin aggregate attributes

[ppittle]

This should be allowed:

                           public class Mixin1
                            {
                                public int Number{ get{ return 42;} }
                            }

                            public class Mixin2
                            {
                                public int OtherNumber{ get{ return 24;} }
                            }

                            [pMixin(Mixin = typeof(Mixin1))]
                            [pMixin(Mixin = typeof(Mixin2))]
                            public class CustomPMixinsAttribute : Attribute
                            {                               
                            }

                            [CustomPMixins]                           
                            public partial class Target
                            {                                
                            }