Folders backup

Hi,

   I love this bundle, it's great for make backups, but I miss something important: Sometimes you want to make a backup with more information than the database. I mean, what is about user uploaded files? (for example). I propose you to add the posibility to select some folders to save in backup. This pull request contains the code to do it: You can add "folders" parameter with an array of folders to backup. Then, if you use --folders (or -F) option when you call "dizda:backup:start" task in the backup the folders will be added.

   What do you think?