Allow pages/posts to be excluded from sitemap

Adds a yaml front matter variable named sitemap_excluded which can be true or false .. if its true then it skips adding that post / page into your sitemap.xml file.

Useful in the case of having a custom 404 page, you don't want that page added to your sitemap.xml

Also adding things to the "EXCLUDED_FILES" array in the plugin feels very awkward, and doesn't work well for pages that are named index.markdown :)