Add SocksV5Command::Other(u8) to allow for extension requests

[wez]

I'm working up some changes to this effect in my fork; I'd prefer to merge them back here if you are amenable.

The high level outline is as below; the names and so on can be tweaked to match your preference.

• Add SocksV5Command::Other(u8)
• SocksV5Command::from_u8 continues to return Option<SocksV5Command> and return only RFC 1928 commands, so that read_request continues to have the same semantics wrt. rejecting unknown commands.
• Add SocksV5Command::from_any_u8(u8) -> SocksV5Command that will populate Other for values outside of RFC 1928
• Add read_request_impl(reader: Reader, allow_extensions: bool) that can opt-in to using SocksV5Command::from_any_u8.
• Adjust read_request to call through to read_request_impl to avoid duplicating the logic

I've tried to avoid making backwards-incompatible API changes in the above, but if you prefer, SocksV5Command::from_u8 could be renamed to SocksV5Command::from_rfc1928(u8) to more closely indicate its underlying action/effect and so on.

Is this something that you'd be interesting in merging? I'll send a PR if that is ok!

[alexkirsz]

Interested :)

And I agree renaming to from_rfc1928 would make more sense. The change can go in a 0.4 release.