Add shuffle plugin (#1443)

* Add shuffle plugin

see #1437

* Change plugin to integrate into nu structure and build system

src/plugins/nu_plugin_stable_shuffle.rs

@@ -0,0 +1,6 @@
+use nu_plugin::serve_plugin;
+use nu_plugin_shuffle::Shuffle;
+
+fn main() {
+    serve_plugin(&mut Shuffle::new());
+}