Added ability to use modules in searchable method

This PR gives you the ability to reference Models from Rails Engines / Modules.

For example:

def searchable_columns
    # list columns inside the Array in string dot notation.
    # Example: 'users.email'
    @searchable_columns ||= [
       "module_or_engine_name/model_name.column_name",
    ]
end

The sortable method seems to work fine using the normal notation (module_or_engine_name_model_name.column_name)