Few improvements to FormPreview, thanks Andy!

This commit adds several new hooks in backwards-compatible way:
 * get_initial to specify initial data based on request
 * get_auto_id to enable different AUTO_ID values
 * get_context for overriding and extending default context contents

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14659 bcc190cf-cafb-0310-a4f2-bffc1f526a37