Support segment contraints in dynamic routes

[testinfected]

Make it possible to enforce a format for a dynamic segment of the route, so we can write:

get("/products/:id", where("id", "[A-Z]{2}-\d{2}")).to(...)

This route would match paths such as /products/AB-123, but not /products/123456.

Anchors will not be supported in the RegExp since we want to match the entire segment.

[testinfected]

RegExp matching would be the default, although other matching styles would be possible by passing a matcher rather than a string.