Add giscus support (#3022)

* Add script in same style as utterances

But adjusted for the various filed differences

* Add initial script

* Add default settings

* Update changelog

* Add feature to readme

* Add comments html

* add comment provider include

* update config in docs

* Add URL for additional reference

* docs for giscus comments

* Unrelated bugfix: add missing version separator

So that things match the "history" doc.

* add space

* update history doc

* update about doc

* add to test config yaml

* remove unnecessary / incorrect async attribute

* probably should pass the right config paths

* lowercase the repo name

* Update docs to address '1' and '0' for reactions_enabled

Figured I'd match the giscus format rather than convert a boolean to an int there.

* update two additional docs

* docs wording fix

README.md

@@ -33,7 +33,7 @@ Minimal Mistakes is a flexible two-column Jekyll theme, perfect for building per
 - Several responsive layout options (single, archive index, search, splash, and paginated home page).
 - Optimized for search engines with support for [Twitter Cards](https://dev.twitter.com/cards/overview) and [Open Graph](http://ogp.me/) data.
 - Optional [header images](https://mmistakes.github.io/minimal-mistakes/docs/layouts/#headers), [custom sidebars](https://mmistakes.github.io/minimal-mistakes/docs/layouts/#sidebars), [table of contents](https://mmistakes.github.io/minimal-mistakes/docs/helpers/#table-of-contents), [galleries](https://mmistakes.github.io/minimal-mistakes/docs/helpers/#gallery), related posts, [breadcrumb links](https://mmistakes.github.io/minimal-mistakes/docs/configuration/#breadcrumb-navigation-beta), [navigation lists](https://mmistakes.github.io/minimal-mistakes/docs/helpers/#navigation-list), and more.
-- Commenting support (powered by [Disqus](https://disqus.com/), [Facebook](https://developers.facebook.com/docs/plugins/comments), Google+, [Discourse](https://www.discourse.org/), static-based via [Staticman](https://staticman.net/), and [utterances](https://utteranc.es/)).
+- Commenting support (powered by [Disqus](https://disqus.com/), [Facebook](https://developers.facebook.com/docs/plugins/comments), Google+, [Discourse](https://www.discourse.org/), static-based via [Staticman](https://staticman.net/), [utterances](https://utteranc.es/), and [giscus](https://giscus.app/)).
 - [Google Analytics](https://www.google.com/analytics/) support.
 - UI localized text in English (default), Arabic (عربي), Brazilian Portuguese (Português brasileiro), Catalan, Chinese, Danish, Dutch, Finnish, French (Français), German (Deutsch), Greek, Hebrew, Hindi (हिंदी), Hungarian, Indonesian, Irish (Gaeilge), Italian (Italiano), Japanese, Korean, Malayalam, Myanmar (Burmese), Nepali (Nepalese), Norwegian (Norsk), Persian (فارسی), Polish, Punjabi (ਪੰਜਾਬੀ), Romanian, Russian, Slovak, Spanish (Español), Swedish, Thai, Turkish (Türkçe), and Vietnamese.