This PR gives users greater flexibility with Excalibur Coroutines!
* Optionally ask coroutines not to start immediately
  ```typescript
  ex.coroutine(function* () { .. }, { autostart: false });
  ```
* New `CoroutineInstance` is returned that is also awaitable
  ```typescript
  export interface CoroutineOptions {
    timing?: ScheduledCallbackTiming;
    autostart?: boolean;
  }
   
  export interface CoroutineInstance extends PromiseLike<void> {
    isRunning(): boolean;
    isComplete(): boolean;
    done: Promise<void>;
    generator: Generator<CoroutineInstance | number | Promise<any> | undefined, void, number>;
    start: () => CoroutineInstance;
    cancel: () => void;
    then: Thenable;
    [Symbol.iterator]: () => Generator<CoroutineInstance | number | Promise<any> | undefined, void, number>;
  }
  ```
  ```typescript
  const co = ex.coroutine(function* () { 
    yield 100;
  });

  await co; // wait for coroutine to finish
  ```

* `start()`/`cancel()`coroutines 

  ```typescript
  const co = ex.coroutine(function* () { 
    yield 100;
  });

  co.start();
  co.cancel();

  await co;
  ```

* nested coroutines! (Nested coroutines do not start running, they are run with their parent)
   
  ```typescript
  const result = ex.coroutine(function* () {
      yield 100;
      yield* ex.coroutine(function* () {
        const elapsed = yield 99;
      });
      yield 100;
  });
  ```