Expose router engine for other implementations

[blakeembrey]

From #13.

Closes #12.

Implements custom path implementations via router inheritance.

Edit: I also still prefer the simplicity of this PR over #22. If you think about it this way it's cleaner - the router engine is the most basic router which takes a function as the "path matcher". Other implementations can build on this to take strings and regexps, etc. The path accepted for route and use can be though of as the "label" for debugging (and backward compat).

[blakeembrey]

@dougwilson Any chance we'll be able to review these sometime? It's been a more than a year (#13) 😮 Otherwise I can look at pulling out the inheritance pieces, I'd love to get a more stable base to continue using (https://www.npmjs.com/package/osprey).