Add secure URL prefix option

The `secureUrlPrefix` option will only apply the middleware to paths that begin with the prefix. I found this handy while working on a couple of apps that served both a JSON API and template partials. Rather than add a path to the `skip` option for each new partial, I instead apply the middleware to the API routes which all begin with `/api`. In this case, the `skip` option can still be used for more granular exemptions. For example, if my API has an authentication route at `/api/authenticate`, I can simply add it to `skip` as before and everything works as expected.