Feature: Support for Async Middleware

[xjamundx]

Let's officially add support for async middleware / routes.

Does this by wrapping all routes at the enrouten level with a promise and catching errors and forwarding them to next(err)

Have been using this internally, but thought it might be cool to make it public. I'd suggest major bumping this and releasing it as a 2.0 enhancement, but outside of the use of the spread operator it should work in node 4+.

@aheckmann @snowinferno @shaunwarman