Added support for SSL Client Authentication.

Added configuration switches:
--ssl-client-certificates-file Location of the client certificate file eventually containing a secret key.
--ssl-client-key-file          Location of a file containing just a secret key. Only needed in case of separate locations of certificate and key.
--ssl-client-key-passphrase    Passphrase for the secret key.

https://github.com/ariya/phantomjs/issues/11275