Allow specifying a custom page initializer (App)

[fs-c]

Since _document.js is already a thing, why not take more inspiration from Next.js?

The default App would be something like

const App = ({ Component, props }) => (
    <Component {...props} />
);

export default App;

where props is the result of the getProps function that a page can optionally export.

Custom Apps would allow for things like

// _app.js
import Header  from 'header'
import Footer 'footer'

const App = ({ Component, props: { headerProps, footerProps, ...pageProps } }) => (
    <Header {...headerProps} />

    <Component {...pageProps} />

    <Footer {...footerProps} />
);

// some/page.js
const Page = ({ posts }) => {
    // ...
};

export const getProps = async () => {
    return { headerProps: { isLanding: false }, footerProps: { dense: true }, posts: [ /* ... */ ] }
};

This is the only feature I can think of that's stopping me from using dhow right now, and I'm willing to take a stab at implementing it. Will probably submit a PR either later today or tomorrow.

[fs-c]

Nevermind, I am so used to working with React and Next.js that I forgot that there is basically no difference between _document.js and _app.js in the given context.

Closing.