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.

[tchoutri]

@Daniel-Diaz Hi! Thanks for the contribution. Would you mind rebase your branch on the current master? :)

[Daniel-Diaz]

@Daniel-Diaz Hi! Thanks for the contribution. Would you mind rebase your branch on the current master? :)

I think I did. Does it look ok?

[tchoutri]

@Daniel-Diaz Fantastic. Would you mind adding it to the test suite? Right here: https://github.com/haskell-servant/servant/blob/master/servant-client/test/Servant/ClientTestUtils.hs#L223

[Daniel-Diaz]

@Daniel-Diaz Fantastic. Would you mind adding it to the test suite? Right here: https://github.com/haskell-servant/servant/blob/master/servant-client/test/Servant/ClientTestUtils.hs#L223

I added an entry to the testing API referenced here.

[tchoutri]

@Daniel-Diaz Excellent, thank you. Would you mind adding a changelog entry like this one?

[Daniel-Diaz]

@Daniel-Diaz Excellent, thank you. Would you mind adding a changelog entry like this one?

I just added the changelog file. Let me know if it looks correct.