title: Cancel RACCommand execution
body: <p>Is there any way to cancel execution of a <code>RACCommand</code>?</p>
  
  <p>For example I have a command with infinite execution signal like this:</p>
  
  <pre><code>RACCommand *command = [[RACCommand alloc] initWithSignalBlock:^RACSignal *(id input) {
      return [RACSignal createSignal:^RACDisposable *(id&lt;RACSubscriber&gt; subscriber) {
          __block BOOL stop = NO;
  
          while (!stop) {
              [subscriber sendNext:nil];
          }
  
          return [RACDisposable disposableWithBlock:^{
              stop = YES;
          }];
      }];
  }];
  </code></pre>
  
  <p>So how can I stop it after calling <code>[command execute:nil]</code>?</p>