Allow more than one parameter per URL segment.

This change adds support for multiple parameters per URL segment. 

Not sure if it goes against any RFC standards or anything, but a while back I made the (questionable?) decision to express image sizes in my app's API as `"/{id}/{width}x{height}"`. Works fine with Web API, but Refit doesn't like it. :bomb: 

I'm working around this in my app's client at the moment with a custom `Resolution` type with a `.ToString()` that serializes it to `"{width}x{height}"`, so if you're not keen on allowing multiple parameters in a segment I'm happy for you to just :hocho: this PR.