(#4)  Add filter function option

Add an option to filter out documents from the search index.  
This is for issue #4.
Yeah, something like this would work fine:

```js
// return true if filtered, false otherwise
function isFiltered(filter, db) {
  try {
    return !!(filter && !filter(doc))
  } catch (e) {
    db.emit('error', e);
    return true;
  } 
}
```

Note that `fun.call(null, arg)` is another optimization killer, plus it's simpler to just do `fun(arg)`. I also think it's wise not to explicitly check for `false`, since some people might return a truthy/falsy value rather than true/false.