Map attributes for import and export

[rap2hpoutre]

ON HOLD. See: #13

Currently, when exporting a model, the columns names are exactly the same as those present in database (minus the ones that are hidden, like password for User model). So exporting a User model returns:

id	name	email
1	raph	raph@example.com

I think we should add something that allows to choose the name of columns.

So calling this:

(new FastExcel(User::all))->mapAttributes([
    'column 1' => 'name',
    'Email' => 'email',
])->export('file.xlsx');

...should create this:

column 1	Email
raph	raph@example.com

When #5 will be done, it should also work for import the same way. So it should be possible to do this to create a bunch of user instances:

$users = (new FastExcel(User::all))->mapAttributes([
    'column 1' => 'name',
    'Email' => 'email',
])->import('file.xlsx');

[rap2hpoutre]

Done for import, via callback:

$users = (new FastExcel)->import('file.xlsx', function ($line) {
    return User::create([
        'name' => $line['Name'],
        'email' => $line['Email']
    ]);
});

Now we have to allow callback on export.

[rap2hpoutre]

Done on export too.