ng-messages: provide the input-element attributes to be used inside of the ng-message template

Would nice to be able to to something like this:

``` html
<div ng-message="minlength">Your input should be at least {{attrs.minlength}} characters long</div>
```

Or even more sophisticated:

``` html
<div ng-message="max"
        ng-if="attrs.type='date'">'The date provided should be no later than the {{attrs.max  |date}}</div>
```

If interested, I could try to provide a pull-request.
