Add screen notification when custom callbacks are added

When defining a custom callback like so:

```
Paratrooper::Deploy.new('amazing-app') do |deploy|
  deploy.add_callback(:before_setup) do
    # code to disable new relic
  end
end
```

There should be the ability to add in a screen notification of what is
happening.

Maybe something like:

```
Paratrooper::Deploy.new('amazing-app') do |deploy|
  deploy.add_callback(:before_setup) do
    deploy.add_screen_notification("Disabling newrelic ping")
    # code to disable new relic
  end
end
```

Thoughts?