title: How to add a validation function to joi.any()?
body: <p>How can I extend <code>joi.any()</code> to add a new rule to it? So that I can use that rule on any existing type, like <code>joi.boolean()</code> or <code>joi.string()</code> for example.</p>
  
  <p>I know how to extend joi with an extension to add a new custom type, but doing it that way I can't combine the new type with existing types.</p>
  
  <p>I'd like to be able to do <code>joi.boolean().myrule()</code> and <code>joi.string().myrule()</code> and for any other existing types. How can I do it? I'm using it with the latest version of joi if it makes a difference.</p>
  
  <p>Is there some way to do <code>joi.any.extend()</code> to add new rules to <code>any()</code> instead of <code>joi.extend()</code> (which adds a new type).</p>