Further plugins, improving the handling of custom paths, adding some more error handling, various fixes.

Adding access checking to every generated path. If no access, path will not be included in the sitemap.

Plugins can now add link options (url parameters and anchors). Sitemap now accepts url parameters and anchors.

Adding new plugin to support the user entity, giving plugins the possibility to define lastmod parameter.

Implementing Drupal 8 plugin support and making sitemap pluggable by other modules providing sitemap links.