Check if torrent exist before adding

[Prototype]

Transmission checks to see if a torrent exist before adding it rejecting the torrent if it's a duplicate. It'd be great if there was a way to check if a torrent already exist through the command line to make scripting more intellegent rather than trail-and-error.

[rb07]

to make scripting more intell[e]gent

It can be done with the current implementation, just use the torrent's hash as an identifier (i.e. extract the hash from the .torrent file, or the magnet link, use transmission-remote to check if that hash already exist...)