Allow changing the "unused suppression comment" lint severity to "error"

Currently it is possible to change the severity of all lints except for the "unused suppression comment" lint. I guess that stems from the fact that suppression linting was developed prior to the concept of other lints and follows a different code-path.

It would be very useful to make it configurable, just as with other lints, e.g.:

```ini
[lints]
unused-suppression=error
```

The main reason for this is that we create test suites for Flow typings, and write code like:

```js
// $FlowErrorExpected
const a : number = 'abc'
```

Where `$FlowErrorExpected` is configured to be a suppression comment.

It makes total sense to have the `flow` command fail here in case the suppression assertion is invalid.

An alternative would be to allow a separate suppression comment, used exactly for the purpose of testing errors, like `$FlowErrorExpected`.