PrivateNetwork type does not match the expected IP addresses

Was there a particular reason behind the RFCs that were chosen in https://github.com/fthomas/refined/pull/356?

I found that `PrivateNetwork` did not match what I expected it to.

The addresses in [rfc5737](https://tools.ietf.org/html/rfc5737#section-4) seem like they should never be routed to:
> 4. Operational Implications
>
> Addresses within the TEST-NET-1, TEST-NET-2, and TEST-NET-3 blocks
> SHOULD NOT appear on the public Internet and are used without any
> coordination with IANA or an Internet registry [RFC2050]. Network
> operators SHOULD add these address blocks to the list of non-
> routeable address spaces, and if packet filters are deployed, then
> this address block SHOULD be added to packet filters.
>
> These blocks are not for local use, and the filters may be used in
> both local and public contexts.

There is also a bug in `Rfc1918ClassBPrivateSpec`. It should start at `172.16.0.0` not `172.15.0.0`. https://tools.ietf.org/html/rfc1918#section-3

I'm by no means an expert but it seems like adding types for [rfc6890](https://tools.ietf.org/html/rfc6890) would be a better way to go.

Without digging into each RFC I suspect what most people would want is any address which is listed in rfc6890 as a valid destination IP address and maybe the loopback addresses.
>Destination - A boolean value indicating whether an address from
the allocated special-purpose address block is valid when used as
the destination address of an IP datagram that transits two
devices.

Thoughts?