feat: TLV support

[g2p]

I added support for TLV extensions, as documented in proxy-protocol.txt

Comes with tests. This will be a version bump, because the Version2 struct now has extra fields.

(FWIW, I think switching the parse side from an "impl Buf" to either Bytes or &[u8] slices (since we probably don't need to support discontinuous data) would make the implementation better by allowing to pass subslices instead of having to add len arguments. But I didn't add that API change.)

Closes #5.

[saiintbrisson]

Hi! I'm wondering whether there have been any updates here yet? I'm using proxy-protocol and TLV support would be really welcome at the moment.