Modularize gulpfile.js file for better extensibility & configurability

[mobidev111]

Breaking up the (already well-structured) gulpfile.js helps to create a better configuration and makes it easier to extend it.

Example:
https://github.com/jtomaszewski/ionic-cordova-gulp-seed/tree/master/gulp

[hassankhan]

https://github.com/cmtt/gulp-di