Add a new "parts" argument

Hi,

I found myself wanting to output durations in the style of "15 minutes ago" or "3 weeks ago" and DateTime::Format::Human::Duration almost did the job.  I've added a new argument to let me do this.  I'm unsure whether I've named or documented this quite right, so you might want to change those, but I think adding a feature to let users show only the "n" most significant units (where "n=1" in my case) would be a good idea.

Thanks for your work on this useful CPAN module,
Tom