Enable client side routing

[agrafix]

The goal of this task is to have a Spock-ghcjs library where we can use in the browser, similar to page.js. reroute already builds with ghcjs.

What to do:

• [ ] identify if there's anything in Spock-core that could be useful in the browser and move that to a shared package
• [ ] reimplement a familiar Spock interface using reroute and the functions from the shared package
• [ ] setup unit testing infrastructure and write some tests
• [ ] build a small demo application (optional)