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>

README.md

@@ -90,7 +90,7 @@ if err != nil {
     <summary>Get r/golang's top 5 posts of all time.</summary>
 
 ```go
-result, _, err := client.Subreddit.TopPosts(context.Background(), "golang", &reddit.ListPostOptions{
+posts, _, err := client.Subreddit.TopPosts(context.Background(), "golang", &reddit.ListPostOptions{
     ListOptions: reddit.ListOptions{
         Limit: 5,
     },
@@ -99,7 +99,7 @@ result, _, err := client.Subreddit.TopPosts(context.Background(), "golang", &red
 if err != nil {
     return err
 }
-fmt.Printf("Received %d posts.\n", len(result.Posts))
+fmt.Printf("Received %d posts.\n", len(posts))
 ```
 </details>