add new dump command

[Dusty Mabe]

On 02/04/2017 12:15 PM, Dusty Mabe wrote:
> 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!

Just realized I forgot to update the docs. If this patch is acceptable
I'll modify docs as well and send a new patchset.

Dusty