Optimize deletes

if you look at [this email from Colin Percival](http://mail.tarsnap.com/tarsnap-users/msg00590.html), you will see that if multiple old archives need to be deleted, we can optimize bandwidth usage and spend less $ by putting multiple <-f archive> arguments on a single invocation of tarsnap (rather than deleting each archive with a separate invocation).

this patch optimizes the functions prune_backups and prune_parts to accomplish that.

i tested the prune_backups function and it works as expected, constructing a valid tarsnap command with multiple -f arguments for each archive that needs deleting. i did not test the prune_parts function, but it uses the same code and logic.

i am running the version of feather in my master branch which includes this patch and my other pull request to fix the regex in the prune_parts function on my production server and all seems well. YMMV of course.