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 https://github.com/brona/iproute2mac/blob/71b4aec1aac76ff97e47e2e9f4770cd1fb8e395a/src/ip.py#L186 we currently do not accept any arguments len(argv) == 1.