Implement `app.defineMiddlewarePhases`

Implement method for registering (new) middleware phases.

 - If all names are new, then the phases are added just before
   the "routes" phase.

  - Otherwise the provided list of names is merged with the existing
   phases in such way that the order of phases is preserved.

Example

    // built-in phases:
    // initial, session, auth, parse, routes, files, final

    app.defineMiddlewarePhases('custom');
    // new list of phases
    //   initial, session, auth, parse,
    //   custom,
    //   routes, files, final

    app.defineMiddlewarePhases([
      'initial', 'postinit', 'preauth', 'routes', 'subapps'
    ]);
    // new list of phases
    //   initial,
    //   postinit, preauth,
    //   session, auth, parse, custom,
    //   routes,
    //   subapps,
    //   files, final

A part of #739, requires https://github.com/strongloop/loopback-phase/pull/5

/to @ritch please review
/cc @raymondfeng 
Good point. I'll land this patch and submit the refactoring in a new PR.