#794 - Scope app middleware to a list of paths

[bajtos]

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)/]
  });

Close #794

/to @raymondfeng @ritch please review

[raymondfeng]

👍