Implement `\Countable` for `\WP_CLI\Iterators\CSV`
Implement `\Countable` for `\WP_CLI\Iterators\CSV`

This makes it possible to start a progress bar for a CSV file:

```php
$iterator = new \WP_CLI\Iterators\CSV( $csv_file );
$progress = \WP_CLI\Utils\make_progress_bar( 'Processing list', count( $iterator ) );
```

The use of `SplFileObject` avoids reading the entire file to memory.
aaemnnosttv/wp-cli,aaemnnosttv/wp-cli,rodrigoprimo/wp-cli,rodrigoprimo/wp-cli,trepmal/wp-cli,trepmal/wp-cli,aaemnnosttv/wp-cli,trepmal/wp-cli,rodrigoprimo/wp-cli