gruntjs - Grunt Usemin: How to define targets? -

i wondering how can use "targets" grunt usemin.
e.g. run sth. like: grunt usemin:app

this tried (error: warning: object # has no method 'indexof' use --force continue.) – same useminprepare

   usemin: {         app : {             html: ['<%= config.app.dist %>/app/{,*/}*.html'],             css: ['<%= config.app.dist %>/app/css/{,*/}*.css'],             options: {                 dirs: ['<%= config.app.dist %>/app']             }         },         site: {             html: ['<%= config.site.dist %>/{,*/}*.html'],             css: ['<%= config.site.dist %>/css/{,*/}*.css'],             options: {                 dirs: ['<%= config.site.dist %>']             }         }     }, 

in grunt usemin targets defined in html page not in gruntfile.

<!-- build:js scripts/vendor.js --> <script src="bower_components/jquery/jquery.js"></script> <script src="bower_components/underscore/underscore.js"></script> <script src="bower_components/backbone/backbone.js"></script> <script src="bower_components/backbone.localstorage/backbone.localstorage-min.js"></script> <!-- endbuild --> 

here scripts/vendor.js target.

source : https://github.com/yeoman/grunt-usemin#the-useminprepare-task