Add support for routing on subdomains

[AlecDivito]

Hi, I created an issue on tide http-rs/tide#690 that was asking about the possibility to adding routing on subdomains as well as on paths. I noticed that it isn't a highly requested feature but I'm using tide for a small app I'm building and need the support to be able to route on subdomains.

However, looking at the code right now, I believe that adding support for subdomains make create some breaking changes. I am just wondering if it would be ok to work on this issue or if there should be more planning happening up front to add this feature into the repository.