Add an option to require client TLS certificates and verification

[johnbelamaric]

This adds an additional TLS flag -client-auth which can take on these values:

• none - the default, do not require client certificates: tls.NoClientCert tls.ClientAuthType
• require - require a cert but don't validate: tls.RequireAnyClientCert
• verify - require and validate client cert: tls. RequireAndVerifyClientCert

Maybe tls.VerifyClientCertIfGiven would be better than the require one here. Or we can just allow them all.

Fixes #41

[johnbelamaric]

This should be followed up with a TLSClientFlags struct in flag.go that provides a separate set of TLS options for use when acting as a client, rather than a server.

[Evgeniy-L]

Closing as this is unlikely to be merged.