Add pass layer

[darkfeline]

This layer adds support for pass, the standard unix password manager.

http://www.passwordstore.org/

[oppenlander]

Hi @darkfeline! I already started a pass layer, in #2784.

This was before a lot of changes to the Spacemacs file layout, so it needs to be updated.
I'll try to get it fixed up later today.

Is there a reason you didn't use the password-store package pass already provides?

[darkfeline]

Nope, just poor research on my part. Let's continue the discussion on your PR.

[robbyoconnor]

Close this PR please.

[robbyoconnor]

👍 thanks :D

This helps keep the project moving efficiently when we have less duplicate PRs/Issues :)