Drush command to check HTTP status of all sitemap paths

I think this would be better to use an external tool (like parallel) to do the status check, but instead focus this on a Drush command to output all of the URLs for a requested sitemap.

This adds a new command for outputting the raw URLs of a sitemap, which would be useful for passing into the parallel command, etc.

Note: I tested this on Simple Sitemap 4.1.6 with 12.5.2.0 with the old function annotation, I reworked it to match the PHP attribute format of 4.2.x but haven't manually tested to confirm it works yet (my codebase is a little out of date for another few weeks).

This is the version that was compatible with 4.1.x.

Fixed #5 X-)

This patch has the correct paths, sorry for the noise.