Provide a mechanism for setting servername

Created by: maxcountryman

Node http.request seems to support this via its options map. However if node-http-proxy also supports this I don't seem to see it. Briefly, this is useful because it may be desirable to proxy requests where the origin server makes use of SNI. While it's trivial to set the Host header appropriately, SNI will fail if the request was not actually made from that host. In my case this is because my server proxies a large number of targets and I would like to be able to preview any changes made to the proxy before asking clients to update their CNAMES.