Add --ssl option for forcing a TLS version

[blyxxyz]

Resolves #120.

[blyxxyz]

---- bad_tls_version_nativetls stdout ----
thread 'bad_tls_version_nativetls' panicked at 'Unexpected success
command=`"D:\\a\\xh\\xh\\target\\x86_64-pc-windows-msvc\\debug\\xh.exe" "--ssl=tls1.1" "--native-tls" "https://tls-v1-2.badssl.com:1012/"`
code=0
stdout=```"HTTP/1.1 200 OK [...]

I'm guessing this is a bug in native-tls's max_protocol_version() on Windows. Might end up disabling the test and printing a warning (on Windows).

[sorairolake]

If possible, I think it's better to add TLS 1.1 or less to possible_values only when native-tls is enabled.

[ducaale]

If possible, I think it's better to add TLS 1.1 or less to possible_values only when native-tls is enabled.

@sorairolake It should be possible via the cfg_attr attribute

    /// Force a particular TLS version.
    ///
    /// "auto" or "ssl2.3" gives the default behavior of negotiating a version
    /// with the server.
    #[cfg_attr(feature = "native-tls", structopt(long, value_name = "VERSION", parse(from_str = parse_tls_version), possible_values = &["auto", "ssl2.3", "tls1", "tls1.1", "tls1.2", "tls1.3"]))]
    #[cfg_attr(not(feature = "native-tls"), structopt(long, value_name = "VERSION", parse(from_str = parse_tls_version), possible_values = &["auto", "ssl2.3", "tls1.2", "tls1.3"]))]
    // An explicit path to Option disables structopt magic we don't want here
    pub ssl: std::option::Option<tls::Version>,

[blyxxyz]

If possible, I think it's better to add TLS 1.1 or less to possible_values only when native-tls is enabled.

You get a more confusing error message that way, so I'd rather not.

[ducaale]

I think If we used our own enum for representing TLS versions, we could have avoided the catch-all arm here plus the Option<std::option::Option> hack. What do you think?

[blyxxyz]

If we do it like that then we need extra effort to ignore TlsVersion::Auto. So I'm not sure it'd end up cleaner overall.

[ducaale]

LGTM