cmd/flatend, flathttp: handler for 404

[johnhorsema]

Suggest to add 404 handler to be configuable under config.toml as follows

[[http.routes]]
code = "404"
service = "notFound"

In Node SDK:

const notFound = async ctx => {
    const fs = require("fs");
    fs.createFileStream("404.html").pipe(ctx);
}