Fix supported browsers config

[LisaFC]

Fixes #120 (I think)

This definitely works locally, though would appreciate a check from someone who knows more about Autoprefixer than I do...

[dholbach]

Checking out your branch, I run into the following problem:

Building sites … You did not set any plugins, parser, or stringifier. Right now, PostCSS does nothing. Pick plugins for your case on https://www.postcss.parts/ and use them in postcss.config.js.

You might need a minimal postcss.config.js like:

module.exports = {
  plugins: [
    require('autoprefixer')
  ]
}

I'm not an expert though.

[coliff]

I think this PR can be closed now. This was fixed in #295.

It now uses Browserslist default config (which you can see here: https://browserl.ist/?q=defaults)