Gulp support

#16 Add gulp to generatored projects

* Added prompt support for selecting a task runner with string argument that defaults to none.  
* Includes a base gulp file that has tasks for jshint/lint (copied .jshintrc from keystonejs project).  

**Notes**
Getting `livereload` will require additional work and will complete in a followup pull-request.  But this at least puts the plumbing in place for people to use gulp.