Support _index_ sitemap item that can be used in case the URL should show the parent sitemap item path

There seems to be some requirement from customers for a sitemap setup where a 'folder' sitemap item is matched, but a child _index_ item must be used.

The functional specs will be as follows. Given the following sitemap:

 + news
      - hst:relativecontentpath = news
      + *
           - hst:relativecontentpath = ${parent}/${1}
           + _index_
                   - hst:relativecontentpath = ${parent}/${1}/index
      + *.html
           - hst:relativecontentpath = ${parent}/${1}

The HST will work as follows:

1) In case a URL is requested that is /news/2010 , then if the folder 2010 contains an document called 'index', then during rendering the _index_ sitemap item is used. Obviously, if the sitemap item _index_ would be

+ _index_
     - hst:relativecontentpath = ${parent}/${1}/foo

instead, then we'd try to find a document (or folder) 'foo', and if it would be

+ _index_
     - hst:relativecontentpath = ${parent}/${1}/${1}

we'd try to find a document (or folder) '2010'

2) In case a URL is requested that is /news/2010 , and there is a _index_ sitemap item, but that relative content path does not point to an existing document (or folder), then just the sitemap item that matches /news/2010 directly is used

3) In case a URL is requested that is /news/2010/mydoc.html , just the **.html matcher is hit

4) Link rewriting for a the document at /news/2010/index will result in the URL /news/2010. A document at /news/2010/09/index will result in the URL /news/2010/09 and the document /news/2010/foo will result in URL /news/2010/foo.html

The above described scenarios will work likewise with * instead of **