Runs all grunt files in sub-folders.

subgrunt-task

// Run sub-grunt files
	grunt.registerMultiTask('subgrunt', 'Run a sub-gruntfile.', function () {
	    var path = require('path');
	    grunt.util.async.forEachSeries(this.filesSrc, function (gruntfile, next) {
	        grunt.util.spawn({
	            grunt: true,
	            args: ['--gruntfile', path.resolve(gruntfile)].concat(grunt.option.flags()),
	        }, function (error, result) {
	            if (error) {
	                grunt.log.error(result.stdout).writeln();
	                next(new Error('Error running sub-gruntfile "' + gruntfile + '".'));
	            } else {
	                grunt.verbose.ok(result.stdout);
	                next();
	            }
	        });
	    }, this.async());
	});