Fix TLS credential options

[bojand]

Should address #55.

All security related options:

-cacert		File containing trusted root certificates for verifying the server.
-cert		File containing client certificate (public key), to present to the server. Must also provide -key option.
-key 		File containing client private key, to present to the server. Must also provide -cert option.
-cname		Server name override when validating TLS certificate - useful for self signed certs.
-skipTLS	Skip TLS client verification of the server's certificate chain and host name.
-insecure	Use plaintext and insecure connection.
-authority	Value to be used as the :authority pseudo-header. Only works if -insecure is used.

Breaking API changes with regards to WithCertificate().
Adds new API functions for additional options.

[srenatus]

Looks good to me! Thank you. 🎈