Add Host API combinator

[Daniel-Diaz]

This combinator allows you to select or restrict APIs according to the provided hosts. For example:

type API = Host "api1.example" :> API1
      :<|> Host "api2.example" :> API2

I implemented it as far as it was needed to make it work, i.e. adding HasClient and HasServer instances. If you want me to add support in more places, let me know.

[arianvp]

Great idea.