add default plugins

Author: yzhou

Sources/ImmutableSwift/generating/Generator.swift

@@ -1,7 +1,12 @@
 class Generator {
     static func Generate(_ datamodel: DataModel) -> String {
         var output = ""
-        let pluginList = datamodel.plugins
+        let pluginList : PluginList?
+        if datamodel.plugins != nil{
+            pluginList = datamodel.plugins
+        } else{
+            pluginList = PluginList(Plugins.defaultPlugins)
+        }
 
         // optional imports
         if datamodel.imports != nil {

Sources/ImmutableSwift/generating/plugins/PluginUtils.swift

@@ -14,4 +14,10 @@ struct Plugins {
         ISCodable.Name: ISCodable(),
         ISCopying.Name: ISCopying(),
     ]
-}
+
+    static let defaultPlugins : [String] = [
+        ISHashable.Name,
+        ISCodable.Name,
+        ISCopying.Name,
+    ]
+}