Backup configuration

I think it is great greed saves backups of files it overrides. But I would like to be able to customize the file name. It would be specially useful to be able to use .filename.bak.0, so that the file is hidden. Let's agree on config syntax.

<pre>
greed {

backup {
   # Replaces the file name format, backup is saved in the same folder as output file.
    fileName = "${GeneratedFileName}.bak.${BackupNumber}"

    # Sets the backup file limit
    fileCountLimit =  3
}

}
</pre>
This reuses the GeneratedFileName that is used for after gen hooks.

It is possible to use folder structures. But backup folder structure needs to be a subfolder of the place where the file is getting generated:

`backup.fileName = "backups/${GeneratedFileName}.${BackupNumber}"`

Works.