Add forward macro to Plug.Router

This pull request adds a `forward` macro to Plug.Router that allows forwarding matched routes to another plug. This is inspired by a utility function in hex_web. I've found this functionality to be generally useful enough to consider including it in Plug.Router.

I'm relatively new to macros, so please let me know if anything looks wacky here.

Also, I'm not sure if I should be providing / forwarding options on [this line](https://github.com/drewolson/plug/blob/router_forward/lib/plug/router.ex#L246) in the `call` invocation.

Feedback welcome.