feat (ng-pluralize): add alternative mapping using attributes

When using ngPluralize, I found myself having to escape the characters " and '.
This PR adds an alternative way to define the mapping using attributes to the <ng-pluralize> element like this

```html
<ng:pluralize count='viewCount' offset='1'
    _0='Nobody is viewing.'
    _1='{{p1}} is viewing.'
    one='{{p1}} and one other person are viewing.'
    other='{{p1}} and {} other people are viewing.'>
</ng:pluralize>
```