Introduce DecodeStreamer for Client.Do

This enables a client to stream values out of the HTTP response. It does
so by providing a result that implements DecodeStreamer with something
like the following.  Error handling has been omitted for clarity.

    // DecodeStream decodes and processes each record of a JSON array
    func (streamingDecoder) DecodeStream(r io.Reader) error {
        dec := json.NewDecoder(r)
        tok, _ := dec.Token() // want: json.Delim('[')
        for dec.More() {
            _ = dec.Decode(&record)
            // do something with record
        }
        tok, _ = dec.Token() // want: json.Delim(']')
        _, err = dec.Token() // want: io.EOF
        return nil
    }

druid.go

@@ -4,6 +4,7 @@ import (
 	"crypto/tls"
 	"encoding/json"
 	"fmt"
+	"io"
 	"io/ioutil"
 	"net/http"
 	"net/url"
@@ -133,6 +134,8 @@ func (c *Client) NewRequest(method, path string, opt interface{}) (*retryablehtt
 	return r, nil
 }
 
+type DecodeStreamer interface{ DecodeStream(io.Reader) error }
+
 func (c *Client) Do(r *retryablehttp.Request, result interface{}) (*Response, error) {
 	resp, err := c.http.Do(r)
 	if err != nil {
@@ -144,6 +147,9 @@ func (c *Client) Do(r *retryablehttp.Request, result interface{}) (*Response, er
 		return nil, err
 	}
 	if result != nil {
+		if decoder, ok := result.(DecodeStreamer); ok {
+			return nil, decoder.DecodeStream(resp.Body)
+		}
 		if err = json.NewDecoder(resp.Body).Decode(result); err != nil {
 			return nil, err
 		}