Improve default post archetypes

[Darthagnon]

Also added "posts.md", which is a copy of "default.md", useful for people who follow the official Hugo docs (https://gohugobrasil.netlify.app/content-management/archetypes/#default-archetypes) and use commands like: $ hugo new posts/my-new-post.md

(cherry picked from commit 803facc29305bc9a16508159f576dde5c7abe2cc)

Fixed version of my earlier pull request. Includes some post metadata that is not currently used by the Diary theme, but which I hope to add support for eventually (multi-authors)

[Darthagnon]

If you prefer, I can remove unused metadata from template

[Darthagnon]

I will be going through the Wiki and adding some parts to the templates, e.g. commented out "disable TOC" option

[AmazingRise]

Hi, Darthagnon I think these default values may be not necessary. And there are also some unused metadata in it.

Anyway, thanks for your contribution.