Allows disabling of hostname verification.

[nicklan]

This PR would allow a user to call: danger_disable_host_name_verification on a ClientConfig to disable host-name verification.

I know this has been sort-of submitted as a PR before and rejected but I do think there are use cases for this and this tries to make it clear that there is a risk to doing this. A few things to consider:

• This forces user to call danger_disable_host_name_verification to make it clear this has risks
• The hyper-openssl has something very similar: https://docs.rs/hyper-openssl/0.2.6/hyper_openssl/struct.OpensslClient.html#method.danger_disable_hostname_verification
• curl has the --insecure option for this

My use case is building a tool that integrates with existing infrastructure, where in some cases the existing things are configured to use a cert that doesn't verify. I imagine this isn't an isolated case (hence all the other tools that support this), and as long as it's very clear what you're doing it's nice to support.

[coveralls]

Coverage decreased (-50.9%) to 42.013% when pulling 2299b0c on nicklan:allow-disable-hostname-verification into dac2274 on ctz:master.

[coveralls]

Coverage decreased (-0.04%) to 92.852% when pulling 23a5b01 on nicklan:allow-disable-hostname-verification into dac2274 on ctz:master.

[ctz]

Thanks! This is effectively (but not exactly) landed on master.