Add parameter overrides to column searchable and sortable

[Blackpig]

In reference to this discussion #51

This PR allows the ->searchable() and ->sortable() methods of the Column to accept a parameter to override a column's default search or sort behaviour.

Main example use case is where a resource column's value is accessed via a model's getXXXXXAttribute where the underlying field does not exist in the table definition. Refer to the above linked discussion for an example use case.

[danharrin]

Hey! Thanks for this. I've just rewritten this as part of #281, with some adjustments...

Originally, sorting on relationship columns was broken. I have fixed this, and you can now able to use multiple sort and search columns for them:

Column::make('customer.name')
    ->searchable(['first_name', 'last_name'])
    ->sortable(['first_name', 'last_name']);