preventDuplicates w/ custom dedupe property

I would like to be able to specify what key I want my flash messages to be de-duped on - sometimes I'll have flashes with the same `message`, but they'll have a different characteristic that means they should still be shown.

eg
```
{ type: 'warning', area: 'maths', message: 'please update your students' }
{ type: 'warning', area: 'reading', message: 'please update your students' }
{ type: 'warning', area: 'maths', message: 'please update your students' }
```

I think this could work well - providing a `dedupe` property
```
{ type: 'warning', area: 'maths', message: 'please update your students', dedupe: `${area}-${message}` }
{ type: 'warning', area: 'reading', message: 'please update your students', dedupe: `${area}-${message}` }
{ type: 'warning', area: 'maths', message: 'please update your students', dedupe: `${area}-${message}` } // should be de-duped
```

`dedupe` could default to `${message}`, Then this could be changed to `dedupe`
https://github.com/poteto/ember-cli-flash/blob/49aec796a5123481f9b546cd53409455f38c0c41/addon/flash/object.js#L12-L13

Would this be a welcome/useful addition? should I write a PR for it?