add source field support

src/py3createtorrent.py

                       dest="private", default=False,
                       help="create private torrent")
 
+    parser.add_option("-s", "--source", type="string", action="store",
+                      dest="source", default=False,
+                      help="include source")
+
     parser.add_option("-c", "--comment", type="string", action="store",
                       dest="comment", default=False,
                       help="include comment")
     #   - piece length
     #   - name (eventually overwrite)
     #   - private
+    #   - source
     # - announce
     # - announce-list (if multiple trackers)
     # - creation date (may be disabled as well)
     if options.private:
         info['private'] = 1
 
+    if isinstance(options.source, str):
+        if len(options.source) > 0:
+            info['source'] = options.source
+
     # Construct outer metainfo dict, which contains the torrent's whole
     # information.
     metainfo =  {
           "  Size:             %s\n"
           "  Pieces:           %d x %d KiB\n"
           "  Comment:          %s\n"
+          "  Source:           %s\n"
           "  Private:          %s\n"
           "  Creation date:    %s\n"
           "  Primary tracker:  %s\n"
          piece_count,
          piece_length / KIB,
          metainfo['comment'] if 'comment'       in metainfo else "(none)",
+         metainfo['info']['source'] if 'source' in metainfo['info'] else "(none)",
          "yes" if options.private else "no",
          creation_date,
          metainfo['announce'],