title: boost program options choices
body: <p>Is there a simple way to check if the argument of an option is inside a set of predefined choices? "Simple" here means without defining an ad-hoc class.</p>
  
  <p>Suppose I have the option <code>--myoption</code> which must have value <code>"myvalue1"</code> or <code>"myvalue2"</code></p>
  
  <p>For example in python is really easy with <a href="http://docs.python.org/library/optparse.html#optparse.Option.choices" rel="noreferrer">choices option in optparse</a></p>