[question] Could `@observe` take a traitlets instance ? 

Right now it takes a name, but could we do:

```
class C(HasTrait):

   foo = FooTrait():

   @observe(foo):
   def _(...):
         ...
```

and extract the name from the foo instance instead of requiring a string ? 

There is probably a reason **not** to do that, but just wondering why. 
