Require Zoom plugin for zoom behavior

Currently if all plugins are removed, the zoom behavior is on by default.  I think it makes sense for it to be off by default, which is the change I added here.

Note that if you want zoom to be on without a button to turn it off, you can use
``` python
plugins.clear(fig)  # clear all default plugins
plugins.connect(fig, plugins.Zoom(button=False))  # no button, zoom on by default
```