Add an API and drush command to scan a directory for managing custom links for static files

Should be obvious from the patch, but if not -- this makes use of the 'subtype' field in {xmlsitemap} to keep track of all the links in a given directory tree. E.g. in my case, I'm invoking this like so:

drush xmlsitemap-custom-scan issues '/\.shtml/'

And ending up with this sort of thing in {xmlsitemap}:

mysql> SELECT id, subtype, loc FROM xmlsitemap WHERE type = 'custom' LIMIT 10;
+----+---------+-----------------------------------+
| id | subtype | loc                               |
+----+---------+-----------------------------------+
| 47 | issues  | issues/59/59.shtml                |
| 48 | issues  | issues/59/editorletter.shtml      |
| 49 | issues  | issues/59/feat-egyptstrikes.shtml |
| 50 | issues  | issues/59/feat-engels.shtml       |
| 51 | issues  | issues/59/feat-food.shtml         |
| 52 | issues  | issues/59/feat-geier.shtml        |
| 53 | issues  | issues/59/feat-iraq.shtml         |
| 54 | issues  | issues/59/feat-lenin.shtml        |
| 55 | issues  | issues/59/feat-shiva.shtml        |
| 56 | issues  | issues/59/feat-tibet.shtml        |
+----+---------+-----------------------------------+
10 rows in set (0.00 sec)

Once the editors import issue #59, we'll remove those files from the filesystem, run the scan again, and those links will disappear from the sitemap (and there will be redirects from the legacy path pointing to the happy new Drupal nodes).

Make sense?

Cheers,
-Derek