Add global DefaultClient (readonly), WithUserAgent option

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

examples/get-top-posts/main.go

@@ -17,22 +17,10 @@ func main() {
 }
 
 func run() (err error) {
-	credentials := &reddit.Credentials{
-		ID:       "id",
-		Secret:   "secret",
-		Username: "username",
-		Password: "password",
-	}
-
-	client, err := reddit.NewClient(credentials)
-	if err != nil {
-		return
-	}
-
 	// Let's get the top 200 posts of r/golang.
 	// Reddit returns a maximum of 100 posts at a time,
 	// so we'll need to separate this into 2 requests.
-	result, _, err := client.Subreddit.TopPosts(ctx, "golang", &reddit.ListPostOptions{
+	result, _, err := reddit.DefaultClient.Subreddit.TopPosts(ctx, "golang", &reddit.ListPostOptions{
 		ListOptions: reddit.ListOptions{
 			Limit: 100,
 		},
@@ -48,7 +36,7 @@ func run() (err error) {
 
 	// The SetAfter option sets the id of an item that Reddit
 	// will use as an anchor point for the returned listing.
-	result, _, err = client.Subreddit.TopPosts(ctx, "golang", &reddit.ListPostOptions{
+	result, _, err = reddit.DefaultClient.Subreddit.TopPosts(ctx, "golang", &reddit.ListPostOptions{
 		ListOptions: reddit.ListOptions{
 			Limit: 100,
 			After: result.After,