Support for adding a redirect with an extension for every page

[fredemmott]

Paraphrasing @satyanash excellent old comment at #214 (comment):

• I want to change all my URIs from /foo/bar.html to /foo/bar/
• Once that's done, for every /foo/bar/ URI, (/foo/bar/index.html) I want to create a new /foo/bar.html URI/file that redirects to /foo/bar/

I have a workaround with a small custom plugin which is pretty straightforward (at least in my site where everything is a Jekyll::Page), but bypassing the GitHub-Pages plugin whitelist is nasty and took much longer to figure out how to do - it would be great if this could be a built-in feature in this plugin, as from the issues here it looks to be a fairly common use case.