Support ugly urls

[kellpossible]

As discussed in #635, it would be nice to have an option similar to Hugo's uglyURLs, to enable pages to be rendered out as /foo.html instead of /foo/index.html, this will have a couple of benefits:

• backwards compatibility when porting another website to zola which uses this url scheme, and for which you wish to keep the old url's alive (I'm currently porting my personal website/blog which was using Pelican and this scheme)
• help support any future efforts to enable for browsing the site on a local file system