Add a way to get the default value of an option from options.

It is not possible to get the default value of a client option using
either its name or instance:

  $options = array(
    'profile' => function ($options, $option) {
      // instance of Predis\Option\OptionInterface
      $profile = $options->getDefault($option);

      // string representing an option handled by $options,
      // returns NULL if the specified name is not handled.
      $profile = $options->getDefault('profile');

      return $profile;
    },
  );

This addition makes it less awkward to get the default value of an
option, especially when not in the context of a callable option
initializer.

examples/MasterSlaveReplicationComplex.php

@@ -51,8 +51,8 @@ $parameters = array(
 );
 
 $options = array(
-    'profile' => function ($options) {
-        $profile = ServerProfile::get('2.6');
+    'profile' => function ($options, $option) {
+        $profile = $options->getDefault($option);
         $profile->defineCommand('hmgetall', 'HashMultipleGetAll');
 
         return $profile;