add 8 additional vsftpd.conf configuration options

The following vsftpd class parameters / vsftpd options are created:
 - $text_userdb_names
 - $listen
 - $max_clients
 - $max_per_ip
 - $pasv_min_port
 - $pasv_max_port
 - $ftp_username
 - $banner_file
Merge pull request #1 from jhoblitt/additional_options

add 8 additional vsftpd.conf configuration options