sample command should support `duration` in same way as play

[emlyn]

~~~Also,~~~ it seems that `sample` doesn't understand the `duration:` opt, is that intentional or an oversight? I think it would make sense for it do be consistent.

[emlyn]

I'm not sure how I missed it, but just had another look and it's in there as clear as day... but the question remains about duration: for sample.