Allow custom dictionary to be passed as an option

I've tweaked the code to allow the user to provide a custom word dictionary, instead of just using traditional Lorem Ipsum. Is this a feature you'd be interested in merging?

Here is an example:

```js
var loremIpsum = require('lorem-ipsum');

var digits = ['one', 'two', 'three', 'four' /* and so on... */];
var hipsterIpsum = ['echo', 'park', 'salvia', 'vhs' /* and so on... */];

console.log(loremIpsum());
// Returns "Anim enim est cupidatat fugiat laborum ea ut laboris consequat proident."

console.log(loremIpsum({words: digits}));
// Returns "Ten two eight six six ten ten eight ten seven three."

console.log(loremIpsum({words: hipsterIpsum}));
// Returns "Trade pork selvage actually belly salvia ethnic vinegar craft pop-up stumptown butcher."


// Credit: Hipster Ipsum taken from http://hipsteripsum.me/.
```

Note: I didn't update CLI usage with the same functionality - currently my use case is as a Node module... though I can take a look at updating CLI code too if you wish.