Make `Tracer.trace` support an optional second argument for ivar

In #16 `IvarTracer` was added, which also targets a specific object. So instead of introducing another `Tracer` method for it, perhaps we can extend `Tracer.trace` instead:

```rb
Tracer.trace(obj) # => uses ObjectTracer
Tracer.trace(obj, :@ivar) # => uses IvarTracer
```

(This idea was provided by @KaanOzkan)