Load options from a file

This small change will allow a file path to be passed as argument from which
the command line options will be loaded.

Extra command line options can be passed (in addition to those already in the
file) and existing ones can be overriden.

Example sshuttle.conf file:
192.168.0.0/16
--remote
[email protected]

Example sshuttle call:
sshuttle @/path/to/sshuttle.conf

Example sshuttle call with verbose flags added:
sshuttle @/path/to/sshuttle.conf -vvv

Example sshuttle call overriding the remote server:
sshuttle @/path/to/sshuttle.conf -r [email protected]

Author: vieira

sshuttle/options.py

@@ -98,7 +98,8 @@ def __call__(self, parser, namespace, values, option_string=None):
 
 parser = ArgumentParser(
     prog="sshuttle",
-    usage="%(prog)s [-l [ip:]port] [-r [user@]sshserver[:port]] <subnets...>"
+    usage="%(prog)s [-l [ip:]port] [-r [user@]sshserver[:port]] <subnets...>",
+    fromfile_prefix_chars="@"
 )
 parser.add_argument(
     "subnets",