Adds a label parameter to the autoexecute command.

Screenshot:
![2014-01-19-135604_590x142_scrot](https://f.cloud.github.com/assets/236201/1949470/1a0500a8-8109-11e3-8179-1e3a81603dd7.png)

Inspired by https://github.com/qbittorrent/qBittorrent/issues/1291.
"%l" will be replaced by the torrent label.