Move the Plug.Router.build_host/1 method to Plug.Router.Utils

I move the above-mentioned method to `Plug.Router.Utils` and renamed it to the more accurate `build_host_match/1`; I also added some doctests.
I also "incidentally" added some explanatory comments to the `Plug.Router.forward/2` macro.
I think we can call it `build_host` and next we can rename `build_match` to `build_path`. What do you think?