Create query strings when constructing paths from routes.

I ran into the need to construct a path with query parameters, rather than just path parameters. Any parameters passed to `Path.build` that aren't explicit path parameters are assumed to be query parameters.

Hope this is of use, I'm not sure this is the cleanest idiomatic approach, though. Always open to suggestions. :)
It won't be optimized. The compiler doesn't know if `param_names/1` have side-effects.