Add autoprefixer for insertion of CSS browser prefixes where needed

[bregenspan]

Issue description

Re: #246 (comment), adding in a CSS autoprefixer would allow clean use of CSS4 features (such as the user-select property in that example).

Note: https://github.com/webpack-contrib/webpack-bundle-analyzer/blob/master/webpack.config.js already uses preset-env with browser versions configured, and already also uses postcss-loader, so this is probably just a matter of adding the autoprefixer to postcss-loader config (https://github.com/postcss/postcss-loader#autoprefixing) and ensuring it uses matching environment settings to the Babel preset.

Technical info

• Webpack Bundle Analyzer version: 3.2.0
• Webpack version: N/A
• Node.js version: N/A
• npm/yarn version: N/A
• OS: N/A