Updates gulp zip task to accept a specific module to build

Allows developers to do this,

```javascript
gulp zip --module cask-angular-dropdown-text-combo
```

This builds only dropdown-text-combo zip file and not everything else. Cleaning everything when we update one single module seems to be redundant.
why not just
```
var directories =  fs.readdirSync('./modules/');
if(argv.module) {
  directories = [argv.module];
}
```
and keep it DRY ;-)