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
 	}