allow to omit front-end stuff

Add '--no-frontend' option which allows to omit the assets folder and front-end-oriented Grunt tasks.
This feature is mentioned in the docs http://beta.sailsjs.org/#/getStarted
>You can also run sails new myCoolApi --no-frontend to omit the assets folder and front-end-oriented Grunt tasks for future projects.  

It works, but I'm not sure I've done it right. Is it ok to skip generators like this? https://github.com/t3chnoboy/sails-generate-new/blob/master/lib/before.js#L36
closes https://github.com/balderdashy/sails/issues/1734