Make DefaultClient a method that returns the global default client

I made this a method to prevent other users from reassigning the
previously exported DefaultClient, e.g. doing something like:

```go
reddit.DefaultClient = nil
```

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

examples/get-subreddit/main.go

@@ -17,7 +17,7 @@ func main() {
 }
 
 func run() (err error) {
-	sr, _, err := reddit.DefaultClient.Subreddit.Get(ctx, "golang")
+	sr, _, err := reddit.DefaultClient().Subreddit.Get(ctx, "golang")
 	if err != nil {
 		return
 	}

examples/get-top-posts/main.go

@@ -20,7 +20,7 @@ func run() (err error) {
 	// 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.
-	posts, resp, err := reddit.DefaultClient.Subreddit.TopPosts(ctx, "golang", &reddit.ListPostOptions{
+	posts, resp, err := reddit.DefaultClient().Subreddit.TopPosts(ctx, "golang", &reddit.ListPostOptions{
 		ListOptions: reddit.ListOptions{
 			Limit: 100,
 		},
@@ -36,7 +36,7 @@ func run() (err error) {
 
 	// The After option sets the id of an item that Reddit
 	// will use as an anchor point for the returned listing.
-	posts, _, err = reddit.DefaultClient.Subreddit.TopPosts(ctx, "golang", &reddit.ListPostOptions{
+	posts, _, err = reddit.DefaultClient().Subreddit.TopPosts(ctx, "golang", &reddit.ListPostOptions{
 		ListOptions: reddit.ListOptions{
 			Limit: 100,
 			After: resp.After,

examples/stream-posts/main.go

@@ -18,7 +18,7 @@ func main() {
 	defer close(sig)
 	signal.Notify(sig, syscall.SIGINT, syscall.SIGTERM)
 
-	posts, errs, stop := reddit.DefaultClient.Stream.Posts("AskReddit", reddit.StreamInterval(time.Second*3), reddit.StreamDiscardInitial)
+	posts, errs, stop := reddit.DefaultClient().Stream.Posts("AskReddit", reddit.StreamInterval(time.Second*3), reddit.StreamDiscardInitial)
 	defer stop()
 
 	timer := time.NewTimer(time.Minute)