Feature Request: Remove Trailing space on truncation

Hello - 

I've come across a needed feature (at least for me) is that when truncating text that falls on a space, I'd like to able to pass in a flag that strips the trailing space before appending the ellipsis, 

```hbs
{{truncate "my string here" 3 true true}}
```

or via options object

```hbs
{{truncate "my string here" 3 true trim=true}}
```
I'd be happy to implement it.
