Using a callback in import

[rap2hpoutre]

Maybe we could use a callback in import:

$result = (new FastExcel)->import('test.xlsx', function($line) {
    return Thing::create([
        'id' => $line['id'],
        'name' => $line['lastname'] . ' ' . $line['firstname'],
    ]);
});

Maybe with this issue, we could avoid making these issues: #7, #2 (for chunk), #5 and even #6 or #4

Maybe after we should add a callback for export too (styling, etc.)