Add html_as_default_extension option to ServeDir

[its-the-shrimp]

Motivation

It's common for file servers to add an .html file extension to the requested path if no file extension is provided, to allow for more concise & human-readable links, e.g. for a request of /dashboard, return /dashboard.html

Solution

Add a html_as_default_extension option to the ServeDir service, which will be true by default, to be in tune with append_index_html_on_directories