Warning if a version transition is being made

[Robert Gildein]

I think there should be a way to warn the user if a new version is installed
that contains major changes. This could be configured in the snapcraft.yaml
file, where the lowest version with warning / info message would be defined.

An example:

```bash
$ snap info prometheus-bind-exporter | grep installed:
installed: 0.2.0-dev (2) 2MB
$ snap refresh prometheus-bind-exporter --stable
WARNING: The old configuration file has been replaced by the snap configuration.
prometheus-bind-exporter v0.4.0-9-g27b19d0 from Jeremy Lounder (jldev) refreshed
```

In this example, I provided a Warning message, as in version 2.0 the
daemon_argument config file was used but in the new 4.0 the snap configuration
(snap get/set) is used.