add new dump command

[Dusty Mabe]

Hi, Thanks for the very useful "pass" program. I'd like to enhance
pass to be able to dump out all passwords in the database to the
screen. Basically like `pass show <pass>` but for all passwords
instead of just a specific one. I created a new "dump" command to
achieve this. I toyed with the idea of implementing this functionality
with `pass show --all` instead of `pass dump` but modifying the
cmd_show() function was going to be a little more tricky to get right.

Let me know what you think!

Dusty