Add ability to prepend middleware

This patch was authored by @jaredh159 and released by @0xTim.

Allows middleware to be inserted at the start of the responder chain. This is useful when working with things like the CORSMiddleware.

Usage:

app.middleware.use(corsMiddleware, at: .beginning)

The default behaviour remains appending:

app.middleware.use(someOtherMiddleware)