Allow to add/overwrite the source field in the info dictionary

[josecelano]

As I described here torrent files can have a source field in the info key dictionary. After implementing the issue #242 we will be storing the original value from the uploaded torrent file.

But some private trackers overwrite this value with a three-letter code identifying the tracker.

The reason is that by changing the source field, you change the torrent infohash and you can avoid the client from adding the tracker URL to a previous torrent if the client already had that torrent.

We could add a new configuration option with the tracker source code or with a generic option to overwrite the source file value when we generate the torrent file. I would keep the original source value in the database and the overwritten version.

cc @da2ce7