Add a filter support to not suppress the undefined and false values in dust references ( may extend it to lists as well )

Nobody has claimed this yet.

In some cases, we would like not to fail gracefully for missing values in the JSON, mostly for validating certain data exists at all times.

Even though this validation can be done on the "context", it is convenient to have a special filter to "enforce" that this value should always exist in the JSON

The only code to be modified is

```
Chunk.prototype.reference = function(elem, context, auto, filters) {
if (typeof elem === "function") {
elem = elem(this, context, null, {auto: auto, filters: filters});
if (elem instanceof Chunk) {
return elem;
}
}
if (!dust.isEmpty(elem)) {
return this.write(dust.filter(elem, auto, filters));
} else {
return this;
}
};
```

Related to : https://github.com/linkedin/dustjs/issues/135

Start at Chunk.prototype.reference and trace how dust.filter handles undefined and false values. Read related issue #135 for context on the requested enforcement behavior. Done means a filter can preserve those values instead of suppressing them, with list handling clarified if it is included.

Written by the indexing model from the issue text.