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)/]
});