Add possibility to pass mysqldump parameters

[kinafu]

For my nextcloud (backup) I need to pass an option (--default-character-set=utf8mb4) to the mysqldump command (in order to properly backup strings containing unicode smileys).

Using our tool databasedump.sh you should be able to pass arguments or environment variables to mysqldump.

If you're curious the nextcloud docs gave me the hint to check, wether one's backup strategy will still work with its new way of storing data in mysql.

[rugk]

Good idea! Should be easy to implement given we already have a specialParams variable.