Support `ip route list SELECTOR`

[brona]

As discussed in #64 with @kbrock it seems that now we can't do e.g. ip route list default, this was likely never implemented rather than a regression.

The list command at this point doesn't accept any arguments, this is as well in the current documentation/help:

#  iproute2mac
Usage: ip route list

# debian
Usage: ip route { list | flush } SELECTOR

As per

iproute2mac/src/ip.py

Line 186 in 71b4aec

any_startswith(["show", "lst", "list"], argv[0]) and len(argv) == 1

we currently do not accept any arguments len(argv) == 1.