Fixes #219 Add argument to allow only updating drafts and prereleases

lib/Inputs.js

@@ -180,6 +180,9 @@ class CoreInputs {
             return undefined;
         return this.name;
     }
+    get updateOnlyUnreleased() {
+        return core.getInput('updateOnlyUnreleased') == 'true';
+    }
     static get omitNameDuringUpdate() {
         return core.getInput('omitNameDuringUpdate') == 'true';
     }