Support function tasks with description

Related to #66, Includes #72 code.
This PR supports function tasks with description using aliases.js.

#72 fixes the error in this case:

```javascript
// aliases.js
module.exports = {
  myAlias: function (target) {
    // Function tasks here...
  }
};
```

And also this adds supporting below:

```javascript
// aliases.js
module.exports = {
  myAlias: {
    description: 'Description of myAlias.',
    tasks: function (target) {
      // Function tasks here...
    }
  }
};

```

I think these function tasks are useful.
Thank you!