Add filter "zero padding"

Another filter I found useful is the zero padding. Would you mind to add it in the library also? Thanks.

```
String.prototype.repeat = function( num ) {
return new Array( num + 1 ).join( this );
}

app.filter('zeroPadding', function () {
return function(input, n) {
if(input === undefined)
input = "";
if(input.length >= n)
return input;
var zeros = "0".repeat(n);
return (zeros + input).slice(-1 * n);
};
});
```

Here is a reference:
https://gist.github.com/njouanin/3348ed708d527de8641c