Basepaths without --basepath

[Rich-Harris]

This is a much simpler solution to #180. Instead of specifying a --basepath flag, just mount the app on a path...

polka()
  .use('/custom-basepath', serve('assets'), sapper({routes}))
  .listen(process.env.PORT);

...and Sapper will figure out the rest. It will infer the basepath from the difference between req.url and req.originalUrl and assign it to req.baseUrl if that doesn't already exist (Express sets this value; Polka doesn't) and handle routing accordingly. It will also set a __SAPPER__.baseUrl property so that the client-side router knows what to do.

To export an app that has been mounted to a base path, you will still need to use --basepath:

npx sapper export --basepath custom-basepath