Split up the Gruntfile initConfig into individual JS files; this gist shows an example based on grunt-contrib-clean. This blog post has a more in-depth look at splitting up the Gruntfile into manageable chunks: http://www.thomasboyt.com/2013/09/01/maintainable-grunt.html

clean.js

// .grunt/config/clean.js
module.exports.tempDir = ["tmp"];

Gruntfile.js

module.exports = function(grunt) {

    var conf = {
        pkg: grunt.file.readJSON('package.json')
    };
    
    grunt.file.recurse('./.grunt/config', function(a, r, s, f) {
        conf[f.replace(/\.js$/, '')] = require('./' + a);
    });

    grunt.initConfig(conf);
};