Adding context to file_url...

...such that a user can additionally specify contexts to the path where the .xml should be saved, outside of the assigned host.

In some situations, we might want the sitemap.xml and sitemaps/sitemap-fragments-*.xml to live elsewhere in our file tree.  The addition of :context upon load Sitemap::Generator.instance.load( :host => "www.somewhere.com", :context => "else/where" ) allows the /sitemaps directory and the sitemap.xml file to live elsewhere.  This also allows for S3 implementation where a developer might house the sitemap.xml and /sitemaps directory on S3.  Sitemap::Generator.instance.load( :host => "s3.amazonaws.com", :context => "some_bucket" ) so long that the real domain :host is passed when defining a literal '/some/path/here', :host => "www.mydomain.com") in the Sitemap::Generator.instance.load { ... } do block.