POC for jazzband#72

Note that if you are using any custom methods on the model for the admin
display, you need to use a History Base class for this to work(jazzband#311).

simple_history/admin.py

@@ -284,3 +284,18 @@ def enforce_history_permissions(self):
         return getattr(
             settings, "SIMPLE_HISTORY_ENFORCE_HISTORY_MODEL_PERMISSIONS", False
         )
+
+
+class SimpleHistoryShowDeletedFilter(admin.SimpleListFilter):
+    title = "Entries"
+    parameter_name = "entries"
+
+    def lookups(self, request, model_admin):
+        return (
+            ("deleted_only", "Only Deleted"),
+        )
+
+    def queryset(self, request, queryset):
+        if self.value():
+            return queryset.model.history.filter(history_type='-').distinct()
+        return queryset