Progress bar incremental changes

Requested in issue #15 

This allows the following:
```php
$progress = $climate->progress(count($items));
foreach ($items as $item) {
    $progress->advance();
}
```

Instead of:
```php
$progress = $climate->progress(count($items));
foreach ($items as $counter => $item) {
    $progress->current($counter);
}
```