Tachyons with webpack boilerplate

Pull requests welcome ✨

Basic set up to for easy customisation for tachyons with webpack.

The idea here is to build tachyons from source. This allows us to:

1. Overwrite colors and media queries
2. Use variables from tachyons in our own custom css
3. Keep the tachyons source out of git history.

How could this be better / What we need to do?

1. We need production config to do minifaction etc
2. We should build a module that uses tachyons-build-css to configure postcss, and exposes the same options, but in a webpack-plugin (I think)..