addes search columns

Hi Ali,
Thanks for your bundle, its very useful!
I wanted to show the search form only for a few columns in my table, so I added a array 'search_columns' and modified the view a bit.
It can be activated using: 
$datatable->setSearchColumns(array('Title', 'Published Date', 'Category'));
I hope you can incorporate this into the bundle so that we can keep using it in our project.
best,
Gerrit