Jekyll: Upgrade to Tailwind CSS 1.2.0, fix purgecss, minor fixes

[olieidel]

Hey!

Thanks for all your work on Tailwind CSS and these setup examples. They're awesome :)

I was working with Jekyll for the past few days and came across a few problems. I fixed all of them and would like to contribute them back to this repository.

This also fixes #56 in which purgecss was not working. I noticed this due to the "generated" site.css being 1.1MB in size, which is the original size. After fixing purgecss, its size is now 8kb - sounds more reasonable.

Major Changes

• Upgrade to Tailwind CSS 1.2.0.
• Fix purgecss: Upgrade purgecss to v2, upgrade other JS dependencies, update extractor regex from Tailwind docs.
• Fix jekyll not generating site: The dependencies jekyll-feed and jekyll-seo-tag were missing, both in the Gemfile and in the _config.yml. Their (liquid) tags were however used in layouts/default.html. So I assume the original intent was to include them and fixed it.

Minor Changes

• Clean up Gemfile: Sync it with what jekyll generates for a new project (jekyll version 3.8.6).
• Clean up exclude list in _config.yml: Order alphabetically, remove unused entries (bin and netlify.toml), add package.json which was included in builds so far.