This small piece of NodeJS code allows you to split in files the Grunt task file. I wrote this because mine was too big, much better to have small files.

Grunt task splitr

---------------------------------------------------------------
Splitr.js
---------------------------------------------------------------

var json = {
    watch: {
        js: {
            files: ['<%= yeoman.app %>/scripts/{,*/}*.js', '<%= yeoman.common %>/js/{,*/}*.js'],
            tasks: ['concat']
        },
        compass: {
            ....
        },
        .....
    }, .......
};

var fs = require('fs');

for (var i in json)
{
    var buffer = JSON.stringify(json[i], null, 4);

    buffer = buffer.replace(/\"([^(\")"]+)\":/g,"$1:");

    buffer = 'module.exports = ' + buffer + ';';

    fs.writeFileSync('./grunt/' + i + '.js', buffer);
}

---------------------------------------------------------------
Gruntfile.js (need to import blod in your package.json file)
---------------------------------------------------------------

'use strict';

module.exports = function (grunt)
{
  require('load-grunt-tasks')(grunt);

    var gruntConfig = {
        pkg: grunt.file.readJSON('package.json')
    };

    grunt.util._.extend(gruntConfig, loadConfig('./grunt/'));

    grunt.initConfig(gruntConfig);

  grunt.registerTask('serve', 'Compile then start a connect web server', function (target) {
    ...
  });

};

function loadConfig(path) {
    var glob = require('glob');
    var object = {};
    var key;

    glob.sync('*', {cwd: path}).forEach(function(option) {
        key = option.replace(/\.js$/,'');
        object[key] = require(path + option);
    });

    return object;
}