Feature idea: Generate 404.html for GitHub Pages when using path-based routing

[felixblaschke]

For the use case "Build Flutter web app and host it on GitHub pages" many users might stumble on the the typical SPA (Single Page Application) issue: web apps like to use path-based routing (http://web.app/deep/link/31) instead of hash-based routing (http://web.app/#/deep/link/31).

Normally accessing a deeplink-url will result in a 404 error. With GitHub pages a used strategy is to copy the index.html to 404.html . (404.html is a github pages magic file)

When using peanut to build web files an optional flag that addresses this issue directly. Alternatively you can provide a "post-build-dart-script" that does this and emphasize it inside readme file / example page.