Pass HTTPS client parameters.

For my application I need fine-grained control over the ciphers used when establishing the TLS sockets underlying the outgoing HTTPS connection. This functionality is supported by the HTTPS client using the `ciphers` parameter:
http://nodejs.org/api/https.html#https_https_request_options_callback
However, I couldn't find a way to use the parameter in the backend of a proxy, hence this pull request.

While I was in there I thought I'd add all of the other optional HTTPS parameters, so this pull request also passes `pfx`, `key`, `passphrase`, `cert`, `ca` and `secureProtocol`.