Add 'gulp help' task using 'gulp-task-listing'

This adds a `help` task to `gulpfile.js`. It is the default implementation using [gulp-task-listing](https://www.npmjs.org/package/gulp-task-listing).

The `help` task lists the tasks supported by this project. It doesn't display any additional information. Still, it can be quite useful in getting acquainted with generator-gulp-webapp and comparing it's options to generator-webapp.