I prepend because I think it is better to regroup all loadNpmTasks together.

If I append, grunt.loadNpmTasks may be mixed with grunt.registerTask like that : 

```
grunt.registerTask('validate', ['csslint']);
grunt.loadNpmTasks('grunt-contrib-csslint');
grunt.registerTask('serve', ['browserSync', 'watch']);
grunt.loadNpmTasks('grunt-browser-sync');
grunt.loadNpmTasks('grunt-contrib-watch');
```

instead of

```
grunt.loadNpmTasks('grunt-contrib-csslint');
grunt.loadNpmTasks('grunt-browser-sync');
grunt.loadNpmTasks('grunt-contrib-watch');

grunt.registerTask('validate', ['csslint']);
grunt.registerTask('serve', ['browserSync', 'watch']);
```