Version config files

[cbaerike]

Some of the default settings don't work for everyone:

• Issue remove database name from dump martin-rohwedder/MySQL-Backup-Manager#28
• Issue Not deleting old backups #22

Fixing these issues is currently complicated because it requires changes to the configuration file. This means we need a version indicator in the file, after which config files can be automatically updated with additional settings by a newer version.