add list of other backup plugin folders to exclude by default

In some cases there can be leftover backup files from other plugins. We should not back those up, plus it can cause issues if total size is too important.

- [x]  [UpdraftPlus](http://wordpress.org/plugins/updraftplus/)
- [ ]  [BackUpBuddy](http://ithemes.com/purchase/backupbuddy/)
- [ ]  [BackWPUp](http://wordpress.org/plugins/backwpup/)
- [x]  [Online Backup for WordPress](http://wordpress.org/plugins/wponlinebackup/)
- [x]  [Duplicator](http://wordpress.org/plugins/duplicator/)
- [ ]  [Xcloner](http://wordpress.org/plugins/xcloner-backup-and-restore/)
- [ ]  WPDBBackup