Common ReWrite request

A common rewrite is to add `.html` to routes on a website.
Eg. `routines/say` is mapped to the web page `routines/say.html` and `types/IO#say` is mapped to `types/IO.html#say`

Currently `:indexes` adds a named file when there is a zero route.

Possibly `:extension<html>` might add `.html` if the route does map to an existing file.
