Support for single-column searching

Thanks for making this; it's been very useful!

I noticed that there wasn't support for single-column searching, which I needed, so I've added it in.

To save you from digging through [the docs](http://www.datatables.net/api): `fnFilter` optionally takes a second argument that specifies the column which the search should target (e.g. `myTable.fnFilter('my search', 3)`), and this adds support for that.