Auto merge of #281 - Ryman:serverdata_macro_typehint, r=cburgdorf
Auto merge of #281 - Ryman:serverdata_macro_typehint, r=cburgdorf

feat(macros): allow hinting the server data type in middleware macro

This can sometimes be required when using some middleware which are
predicated on the datatype of the Server. An alternative would be to
litter the handler with extra type annotations (which is awkward without
type ascription), or to use explicit type-annotated functions as middleware.

Example usage:
```
middleware! { |res| <ServerData>
   // res is of type Response<ServerData>
}
```

Ryman/nickel.rs,Ye-Yong-Chi/nickel.rs,BenKnigge/nickel.rs,nickel-org/nickel.rs,jolhoeft/nickel.rs,anvie/nickel.rs