This format is better supported by crawlers than the previous txt "one
line per URL" format. It also has more features but we are not using
them yet.

The escapeURL function allows to escape the Location field. This prevents
URLs from containing illegal characters by converting them to their
percent encoding representation.

It is implemented as a text/template instead of using the encoding/xml
package, because we do not need all of the xml features and we only need
to encode it and never to decode it. It is a lot simpler this way.