enable cargo build --features=extra to build plugins (#448)

src/plugins/nu_plugin_core_example.rs

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

src/plugins/nu_plugin_core_inc.rs

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

src/plugins/nu_plugin_extra_gstat.rs

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