support custom PostCSS / Babel configurations

[thescientist13]

Type of Change

• New Feature Request
• Documentation / Website
• Improvement / Suggestion
• Bug
• Other (please clarify below)

Summary

Currently there is no way to extend Greenwood's built in Babel and PostCSS configuration, to either add custom plugins or additional stages, as seen in #363 .

Details

In webpack.config.common.js the relative config files for both are provided and loaded. We should update this to

1. Detect if equivalent file is found in users project directory
2. Use that version instead
3. Document in the website the new behavior
4. Add a test (I suppose detecting transpiled features in the output would have to the be test?)

Only immediate thought that comes to mind that might make this tricky is that both assume the relative location of .browserslistrc which may differ now when the Babel and PostCSS files are loaded from a different location now. Not sure if .browserslistrc path can be hardcoded or configured on the fly in that case? Seems like this enhancement could / should be done along with #224 ?