Add support for IPv6 addresses and routes

Add parameter ifaddr6 to netif::interface. When given an IPv6
address, adds that address as the main IPv6 address for the
interface.

Add parameter routes6 to netif::interface. When given an array
of IPv6 (address, gateway), adds routes to IPv6 routing table.
Add aliases, aliases6 params to netif::interface

Add support for inet and inet6 alias addresses by passing an
array of address via either aliases => [ ... ] or
aliases6 => [ ... ]
Update README with aliases, aliases6 examples
Eliminate dynamic lookup of $netif::onparent
Merge pull request #1 from tangledhelix/feature/dynamic-lookup

IPv6 and alias changes/updates and one bugfix