Provide cron regeneration interval setting

Currently there is a setting to regenerate the sitemap on every cron run. If you don't want to regenerate the sitemap on every cron run, but only every x hours, you have to use drush or do it manually. Generating the sitemap may take a lot of system resources, so you may not want to do it every time, especially since (afaik) the sitemap isn't checked as often by search engines even for sites with a high page rank. Since drush may not be available on every system and relying on manual execution is prone to human error, I propose to add a setting to optionally define a cron regeneration interval, that would regenerate the sitemap only, if x hours elapsed since the last generation. By default, the sitemap would still be regenerated on every cron run.