When the command is a function, call it to get the executable command.

If a function is provided for `command`, call that function passing in the reference to the running grunt-shell task. This allows for more flexible multi-step scripts to run, passing data from the output of one to the next, using a closured object.

### Example Usage

This example runs an ant script, that needs various AWS credentials, which it reads from the command line in the first grunt-shell task, then uses in the second.

```javascript

/*global module:false*/
module.exports = function(grunt) {
	'use strict'
	var aws_id;
	var aws_secret;
	var fs = require('fs');
	
	/**
	 * Get access info for AWS
	 */
	function grabEnvVars(err, stdout, stderr, cb){
		var vals = stdout.split("\n");
		aws_id = vals[1].split("= ")[1];
		aws_secret = vals[2].split("= ")[1];
		cb();
	}

	/**
	 * Create the command to run Ant with credentials.
	 * With grabEnvVars, the id and secret are in the closure.
	 */
	function getRepFiles(){
		var rep_files = fs.readdirSync('/list/of/rep/files');
		var output = rep_files.map(function(file){
			// Set vars that Ant will use.
			var env_vars = 'AWS_ACCESS_KEY_ID='+aws_id+' AWS_SECRET_ACCESS_KEY='+aws_secret+' ';
			return env_vars + 'ant -buildfile ./path/to/build.xml -Dlog.level=INFO -Drep.file='+file;
		}).join(' && ');
		return output;
	}

	grunt.initConfig({
		shell: {
			// Call the grabEnvVars to load into the closure scope.
			setUpEnv: {
				command: 'gpg --decrypt /var/some/keyfile | grep "personalization.dev.writer"',
				options: {
					callback: grabEnvVars,
		  			failOnError: true
				}
			},
			// Using the setUpEnv task that loaded the variables, use getRepFiles.
		  	woth: {
				command: getRepFiles,
				options: {
					stdout: true,
					stderr: true,
					failOnError: true
				}
			}
		}	  	
  	});

	grunt.loadNpmTasks('grunt-shell');

	grunt.registerTask('default', ['shell:setUpEnv', 'shell:woth']);
};
```