Feature: Add API for configuring a base path for routing

Most often seen with (but not limited to) certain hosting providers like GitHub Pages, users have to do a fair bit of work to set up a base path for their app as our router will use the entire `location.pathname` in most places.

Adding a `base` (or `basename`, tbd) API option would simplify user code considerably in these situations and shouldn't be too hard for us to support.