exception where af is specified with no other arguments

[bsholdice]

I get a python exception when I specify the address family but forget an argument.

e.g.

bsholdice@mbp ~/Code/others/iproute2mac/src ±master » ./ip.py -6                                            1 ↵
Traceback (most recent call last):
  File "./ip.py", line 553, in <module>
    main(sys.argv[1:])
  File "./ip.py", line 55, in inner
    if not func(*args, **kwargs):
  File "./ip.py", line 535, in main
    if argv[0] == '-V':
IndexError: list index out of range 

obviously I mean to include an argument (ip -4 a or ip -4 r etc) so this is definitely a user error. But I do it so frequently I thought I'd open PR #25 to handle it.

Thanks for writing this wrapper. very handy!

[brona]

Merged and about to cut a release