Implement basic player plugin

src/plugins/mod.rs

@@ -0,0 +1,10 @@
+use crate::prelude::*;
+
+pub struct PlayerPlugin;
+
+impl Plugin for PlayerPlugin {
+    fn build(&self, app: &mut AppBuilder) {
+        app.add_startup_system(add_player.system())
+            .add_system(greet_player.system());
+    }
+}