Feature request: combine callback calls after holding notifications

In the following example:

```python
from traitlets import HasTraits, Integer

class State(HasTraits):
a = Integer()
b = Integer()

def callback(*args, **kwargs):
print(args, kwargs)

s = State()
s.observe(callback)
with s.hold_trait_notifications():
s.a = 1
s.b = 2
```

Callback gets called twice. However, it would be nice to have a way to minimize the number of callback calls when exiting the context manager.

The reason I ask about this is because I would like to consider transitioning [glue](http://www.glueviz.org) from using [echo](https://github.com/glue-viz/echo), which is our own trait-like library by traitlets. The case I describe above is common in our code, where we change a bunch of traits and then want to e.g. force a redraw (but only one and not one per trait).