Add an ellipsis option to the truncate function

Hi Henrik,
A simple modification to add ellipsis text at the end of the truncated text.
Now you can have:

> Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore mag…
> Read more >

by using

```
$('.my-text').truncate({
  ellipsis: '…',
  more: 'Read more >',
  link_prefix: '<br>',
  link_suffix: ''
});
```
I also updated the README with this ellipsis option and added the other missing options (css_more_class, css_less_class, …)

Let me know what you think…