Add navigation static links options

Added RailsAdmin::Config.navigation_static_links_options

Example usage:
```ruby
RailsAdmin.config do |config|
  config.navigation_static_links = {
    'Test Link' => 'http://www.google.com'
  }
  # will be added to all navigation static links
  config.navigation_static_links_options = {
    :rel => 'nofollow'
  }
end
```