Use a separate file for getting document

[egoist]

I'm considering adding an SPA target to Ream so that you can build the app as a regular client-side SPA.

But currently you can only set custom document markup using the document method in the entry file: https://ream.js.org/guide/custom-document.html which means it's only available when using SSR, so it would better if you can set document using a index.html with ejs syntax support.