Support base paths

[Rich-Harris]

Use case: you want to serve a Sapper app from somewhere other than the root of your website. On the server this is relatively straightforward:

import polka from 'polka';
import sapper from 'sapper';
// ...
import { routes } from './manifest/server.js';

const app = polka();
const subapp = polka();

// ...

subapp.use(sapper({
  routes
}));

app.use('/subapp', subapp);

app.listen(4567);

This will (or should — I haven't actually tested it) mount e.g. routes/foo.html to /subapp/foo.

All that's missing, in theory, is for the client-side app to know about the base path. Proposed API:

import { init } from 'sapper/runtime.js';
import { routes } from './manifest/client.js';

init(document.querySelector('#sapper'), routes, {
  base: 'subapp'
});