Add disable keyboard parameter (disablekb)

Problem/Motivation

In relation to accessibilty and being WCAG compliant it's important to disable the shortcut keys with adding the disablekb parameter. See https://developers.google.com/youtube/player_parameters#disablekb. Is it possible to add this parameter to the YouTube video field settings? The attached patch is a quick fix that works for me and hardcodes this parameter in the template_preprocess_youtube_video function.