introducing base_path option

This should complement the `:origin` option, in order to provide good
defaults to build REST SDKs around of.

Ex:

```ruby
HTTPX.with(origin: "https://api.this-product.com", base_path: "/v3.1")
```

lib/httpx/options.rb

@@ -132,6 +132,7 @@ module HTTPX
     def freeze
       super
       @origin.freeze
+      @base_path.freeze
       @timeout.freeze
       @headers.freeze
       @addresses.freeze
@@ -141,6 +142,10 @@ module HTTPX
       URI(value)
     end
 
+    def option_base_path(value)
+      String(value)
+    end
+
     def option_headers(value)
       Headers.new(value)
     end

lib/httpx/request.rb

@@ -49,7 +49,9 @@ module HTTPX
         origin = @options.origin
         raise(Error, "invalid URI: #{@uri}") unless origin
 
-        @uri = origin.merge(@uri)
+        base_path = @options.base_path
+
+        @uri = origin.merge("#{base_path}#{@uri}")
       end
 
       raise(Error, "unknown method: #{verb}") unless METHODS.include?(@verb)

test/support/requests/get.rb

@@ -20,6 +20,14 @@ module Requests
       verify_body_length(response)
     end
 
+    def test_http_get_option_origin_base_path
+      status_uri = URI(build_uri("/status"))
+      http = HTTPX.with(origin: status_uri.origin, base_path: status_uri.request_uri)
+      response = http.get("/200")
+      verify_status(response, 200)
+      assert response.uri.request_uri == "#{status_uri.request_uri}/200"
+    end
+
     def test_http_request
       uri = build_uri("/get")
       response = HTTPX.request(:get, uri)