Custom selector and fix for change event

859a7ae fixes your todo item at the bottom. It shouldn't affect speed since nothing in the loop changed.

01f76c5 adds a custom selector option, similar to what was suggested in #6.

I've modified the code for clarity and tested it on a list of about 2,100 items. Compared to the original version, there appears to be no noticeable difference when a selector is not specified (i.e. the default, backwards-compatible behavior). As expected, however, there is a nominal performance hit when using a selector. (I've noted this in the readme for prospective users.)

I believe this will be useful for a lot of people. My use case is something like this:

```html
<ul>
    <li>
        <span class="badge">Badge Text</span>
        <span class="description">Description</span>
    </li>
</ul>
```

By default, the filter will compare against `Badge TextDescription`, which is undesirable. But when `.description` is used as a selector, the plugin compares only against `Description`, which is the desired behavior.

7ad6fcd removes the use of `addback()`, which was benign but unnecessary and would have made the plugin incompatible with jQuery < 1.8.