Make Command accessible from Context

In a project I'm working on, I've created a graceful way to recover from failed arguments with a nice message for the user, such as this:

```go
func requireArguments(c *cli.Context) {
  // argument tests
  if err {
    fmt.Println("For more information, run: binary " + c.Command.Name + " --help") 
    os.Exit(1)
  }
}
```

I believe Context should have access to the Command that was called by the user.  This is a pull request implements this in a simple style I saw possible.  Cheers! Chris