Add option for automated JS discovery

[mehigh]

Getting inspiration from this gist and using glob-fs, we could implement an option to load as entries all main.js files that appear under the provided folder structure as an option.
CC @dero / @DamianSam

var files = glob.readdirSync('./app/Resources/assets/js/**.js');
// Use each entry to build the key and add them to entries
var entries = {};
for (var i = 0, l = files.length; i < l; i++) {
    var key = files[i].replace('app/Resources/assets/js/','').replace('.js', '');
    entries[key] = source_dir + '/js/' + key;
}