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
user@example.com

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 otheruser@test.example.com

sshuttle/options.py

@@ -98,7 +98,8 @@ class Concat(Action):
 
 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",