v2.5.8: updated Sitemap class

- render protection (don't render sitemap with >50000 elements)
- option to force using a size limits for the number of elements in a
sitemap in the store() method, instead of default splitting of bigger
sitemaps to smaller sitemaps with sitemapindex
- memory leak fix (caching for file generation, clear unneeded
resources)
- fixed wrong file extension of html/txt generated files