Let yeoman deliver a custom message.

This is a possibly terrible and gross implementation of custom greeting messages from yo.

```js
this.greet('Hey! This is my awesome generator!');
```

...

```bash
     _-----_
    |       |
    |--(o)--|   .--------------------------.
   `---------´  |  Hey! This is my awesome |
    ( _´U`_ )   |        generator!        |
    /___A___\   '__________________________'
     |  ~  |
   __'.___.'__
 ´   `  |° ´ Y `
```

Try it out!