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.
Update markdown files to use relative links.

See https://help.github.com/articles/relative-links-in-readmes for details.

[ci skip]