Word truncation

Patched the truncate strings method to include truncation by word

Examples: 

    'Once upon a time'.truncate(4, word: true)
    => "Once upon a time"

    'Once upon a time in a world far far away'.truncate(4, word: true)
    => "Once upon a time..."

    'Once<br>upon<br>a<br>time<br>in<br>a<br>world'.truncate(5, word: true, separator: '<br>')
     => "Once<br>upon<br>a<br>time<br>in..."

    'Once<br>upon<br>a<br>time<br>in<br>a<br>world'.truncate(5, word: true, separator: '<br>', omission: '[...]')
     => "Once<br>upon<br>a<br>time<br>in[...]"
I don't think it's sufficiently useful.

(Also, the '(Read more) ...' example seems at best misleading... if you're passing a different omission than you gave to `truncate`, things are surely going to get confusing?)