[output] Add a minimal output option for the middleware

This adds another option for output to the middleware giving you the....
```
webpack: bundle is now VALID/INVALID
```
... messages without the full output of the compilation stats.

It's a middle ground between the default and the `noInfo` and `quiet` options.

I wonder if a better general solution would be to add the concept of a `logLevel` option, and have something like...

```
logLevel: "QUIET" // -> Currently quiet
logLevel: "WARNINGS" // -> Currently noInfo
logLevel: "MINIMAL" // -> Currently minimalOutput
logLevel: "DEBUG" // -> Currently the default
```

I can investigate that if you're interested.