Add global DefaultClient (readonly), WithUserAgent option

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

reddit/reddit-user-agent.go

@@ -18,6 +18,8 @@ func cloneRequest(r *http.Request) *http.Request {
 }
 
 // Sets the User-Agent header for requests.
+// We need to set a custom user agent because using the one set by the
+// stdlib gives us 429 Too Many Requests responses from the Reddit API.
 type userAgentTransport struct {
 	userAgent string
 	Base      http.RoundTripper