Use a separate file for getting document
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](https://lodash.com/docs#template) syntax support.