Introduce an UrlGenerator

[shrink0r]

We need an idea on how to tackle url-generation.
FastRoute is being used for the matching, but it probably isn't sufficient for generating urls, as it doesn't support named routes and due to the way it handles optional parameters.
@ddelbondio Happy to hear/see suggestions. :)

[MrHash]

This seems to be an open issue nikic/FastRoute#66 but they do suggest that named routes is a trivial extension nikic/FastRoute#11 although unlikely that will give the kind of flexibility needed..