Support different url modifiers

[sckott]

port, scheme, etc.

[sckott]

right now like

HttpClient$new(url = "https://httpbin.org")$get(path = "get")

possibly allow different way to set these:

HttpClient$new(scheme = "https", base = "httpbin.org", port = 8983)$get(path = "get")

but seems faraday and requests both don't do this fine grain url setting other than for proxies, so maybe not, leave open for now