Add setting to prefer clean URLs over index.html

[chabala]

I'd like to configure sitemap generation to list clean URLs in cases where the default index page is found.

E.g.:

Current behavior	Proposed behavior
https://example.com/index.html	https://example.com/
https://example.com/path/index.html	https://example.com/path/
https://example.com/other.html	https://example.com/other.html (no change for non-index pages)

I'd prefer this as the default behavior, but it could be a configurable option.

Google Search Console (the consumer of the sitemap.xml) treats the clean URL and the index.html URL as distinct pages, and I want the clean URL to be canonical, and listed in the sitemap.

[chabala]

Thank you, these changes look good. I'm going to test out the new version.