updating tailwind to 1.0

[allanwhite]

I've updated Tailwind to 1.x here. More detailed upgrade instructions can be found on their site.

• Renamed tailwind.js => tailwind.config.js
• Updated references in postcss.config.js
• Added parts of the original readme from the middleman stack repo you referenced; if you don't want to include it, it's fine; found parts of it useful and I think others would too.
• Added, but did not activate, tailwind-css-variables. It adds Tailwind variables as CSS custom properties as a list to your project. Lots of other useful plugins as well out there.

This is a very useful stack! Thanks for your work - much appreciated.