Duplicate code for generating sitemaps from CLI and cron

[nickdaugherty]

CLI and cron duplicate code used to generate the sitemap for a given day:

$date_stamp = Metro_Sitemap::get_date_stamp( $year, $month, $day );
if ( Metro_Sitemap::date_range_has_posts( $date_stamp, $date_stamp ) ) {
    Metro_Sitemap::generate_sitemap_for_date( $date_stamp ); // TODO: simplify; this function should accept the year, month, day and translate accordingly
}

This should be refactored into a method that includes the date_range_has_posts check, as we need to attempt to delete the sitemap if it doesn't have any posts, and there is no need to maintain that code in multiple places.

Needed for #63