Revamp listing decoding, include after/before anchors in response

Now, instead of returning an object containing a list of results + the
anchors, we return just the list. The anchors are available in the
response object. Much cleaner this way in my opinion

go-github and godo do it this way too. They include some meta
information in the returned response objects

Signed-off-by: Vartan Benohanian <vartanbeno@gmail.com>

reddit/reddit.go

@@ -269,6 +269,12 @@ func (c *Client) NewRequestWithForm(method string, path string, form url.Values)
 // Response is a Reddit response. This wraps the standard http.Response returned from Reddit.
 type Response struct {
 	*http.Response
+
+	// Pagination anchor indicating there are more results after this id.
+	After string
+	// Pagination anchor indicating there are more results before this id.
+	// todo: not sure yet if responses ever contain this
+	Before string
 }
 
 // newResponse creates a new Response for the provided http.Response.
@@ -277,6 +283,11 @@ func newResponse(r *http.Response) *Response {
 	return &response
 }
 
+func (r *Response) populateAnchors(a anchor) {
+	r.After = a.After()
+	r.Before = a.Before()
+}
+
 // Do sends an API request and returns the API response. The API response is JSON decoded and stored in the value
 // pointed to by v, or returned as an error if an API error has occurred. If v implements the io.Writer interface,
 // the raw response will be written to v, without attempting to decode it.
@@ -310,6 +321,10 @@ func (c *Client) Do(ctx context.Context, req *http.Request, v interface{}) (*Res
 				return nil, err
 			}
 		}
+
+		if anchor, ok := v.(anchor); ok {
+			response.populateAnchors(anchor)
+		}
 	}
 
 	return response, nil