Add nobackup-file preference.

Directories containing a file with this name will not be backed up. I find it easier to drop a .nobackup file in directories I don't want backed up instead of maintaining a file of exclusions. This is also useful for scripts that create data in the repository but don't want it to be backed up.

src/duplicacy_preference.go

@@ -22,6 +22,7 @@ type Preference struct {
 	BackupProhibited  bool              `json:"no_backup"`
 	RestoreProhibited bool              `json:"no_restore"`
 	DoNotSavePassword bool              `json:"no_save_password"`
+	NobackupFile      string            `json:"nobackup_file"`
 	Keys              map[string]string `json:"keys"`
 }