Add Orchestra\Html\Table\Grid::searchable() and Orchestra\Html\Table\Grid::sortable() to facilitate searching and sorting.

Solve #21 

```php
Table::of('users', function ($table) {
    $table->with(User::newQuery(), true);

    $table->searchable(['email', 'fullname'], 'q'); // would search either both field for "crynobone%" when using ?q=crynobone*

    $table->sortable('order', 'sort'); // will sort by "email" ascending when given ?order=email&sort=asc

    $table->column('email');
    $table->column('fullname');
});
```