Color support

Adds simple color support with the help of [Chalk](https://github.com/sindresorhus/chalk).

Example usage:
```javascript
asciify('ASCIIFY!', {color: 'green'}, function (err, result) {
	console.log('\nASCIIFY!\n' + result);
});
```

Produces the following output:
![screen shot 2013-11-13 at 13 54 31](https://f.cloud.github.com/assets/426143/1531358/3747e7a0-4c63-11e3-87b0-2df152b76be8.png)