Feature: Add API for configuring a base path for routing

[rschristian]

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.