Scope app middleware to a list of paths

Add a new argument to `app.middleware` allowing developers
to restrict the middleware to a list of paths or regular expresions.

Modify `app.middlewareFromConfig` to pass `config.paths` as the second
arg of `app.middleware`.

Examples:

    // A string path (interpreted via path-to-regexp)
    app.middleware('auth', '/admin', ldapAuth);

    // A regular expression
    app.middleware('initial', /^\/~(admin|root)/, rejectWith404);

    // A list of scopes
    app.middleware('routes', ['/api', /^\/assets/.*\.json$/], foo);

    // From config
    app.middlewareFromConfig(
      handlerFactory,
      {
        phase: 'initial',
        paths: ['/scope', /^\/(a|b)/]
      });