enable configuring of nodejs https options

Hi,

I noticed that there is currently no possiblity to provide additional configuration for https requests in nodejs.

See the nodejs manual here: http://nodejs.org/docs/latest/api/https.html#https_https_request_options_callback

With this pull request, it is possible to configure the extended tls.connect configuration options. I needed this because I wanted to to a request against a https backend without a valid ssl cert. Now I can do a request with the `rejectUnauthorized` configuration option like in this example

```js
client({ 
  path: '/data.json',
  httpsOptions: {
    rejectUnauthorized: false
  }
}
).then(...
```