add plugins

cargo fmt

src_testbed/plugin.rs

@@ -10,3 +10,10 @@ pub trait TestbedPlugin {
     fn draw(&mut self);
     fn profiling_string(&self) -> String;
 }
+
+pub trait HarnessPlugin {
+    //FIXME: is run_callbacks needed?
+    fn run_callbacks(&mut self, physics: &mut PhysicsState, t: f32);
+    fn step(&mut self, physics: &mut PhysicsState);
+    fn profiling_string(&self) -> String;
+}