Ability to rename files on save

[hugofabricio]

I added a 'new_filename' option to rename the file before saving when this option exists in the method hasAttachedFile();

Examples:

        $this->hasAttachedFile('avatar', [
            'new_filename' => md5(strtotime('now')),
            'styles' => [
                'thumb' => '240x240'
            ],
        ]);

        $this->hasAttachedFile('avatar', [
            'new_filename' => 'newname',
            'styles' => [
                'thumb' => '240x240'
            ],
        ]);

[hugofabricio]

What do you think?