Fix #117 to allow for no subnets via file (-s)

This should fix an issue introduced in #117 where when no subnets are
given via file (-s file) the variable is None instead of an empty list
and the concatenation with the subnets given as positional parameters
fails.

sshuttle/options.py

@@ -270,6 +270,7 @@ def __call__(self, parser, namespace, values, option_string=None):
     metavar="PATH",
     action=Concat,
     dest="subnets_file",
+    default=[],
     type=parse_subnet_file,
     help="""
     file where the subnets are stored, instead of on the command line