Would be nice to have $count parameter inside plural translate directive

Now plural directive looks like:
```html
 <span translate translate-n="some.deep.object.length" translate-plural="There are {{some.deep.object.length}} messages">There is {{some.deep.object.length}} message</span>
```
Which is not great - because you need to copy all this long string "some.deep.object.length" inside you translations.
It would be handy to have some special variable '$count' inside plural directive like it done for ngRepeat directive.
Than our code looks like:
```html
 <span translate translate-n="some.deep.object.length" translate-plural="There are {{$count}} messages">There is {{$count}} message</span>
```