Add a credential helper for gopass

Background Information

gopass is a rewrite of pass written in Go. It provides a very similar interface to zx2c4's pass, while adding some additional functionality that helps using the password manager within team or multi-team environments, such as mounting external stores, encrypting different blobs for different recipients, etc.

It has a configuration file, located at $XDG_CONFIG_HOME/gopass/config.yml, example below:

root:
  askformore: false
  autoclip: true
  autoimport: true
  autosync: true
  check_recipient_hash: false
  cliptimeout: 45
  concurrency: 1
  editrecipients: false
  nocolor: false
  noconfirm: false
  nopager: false
  notifications: true
  path: gpgcli-gitcli-fs+file:///home/username/code/src/github.com/username/my-personal-store
  recipient_hash: {}
  safecontent: false
  usesymbols: false
mounts:
  work:
    askformore: false
    autoclip: true
    autoimport: true
    autosync: true
    check_recipient_hash: false
    cliptimeout: 45
    concurrency: 1
    editrecipients: false
    nocolor: false
    noconfirm: false
    nopager: false
    notifications: true
    path: gpgcli-gitcli-fs+file:///home/username/code/src/git.company.com/username/my-work-store
    recipient_hash: {}
    safecontent: false
    usesymbols: false

The root dictionary, above, holds the configuration for the root ("default") store. The mounts list provides zero or more dictionaries for additional stores which are "mounted" at the top-level name (the mount's name). For example, with the above configuration:

$ gopass foo

would attempt to access and decrypt foo.gpg in /home/username/code/src/github.com/username/my-personal-store, and

$ gopass work/foo

would attempt to access and decrypt foo.gpg in /home/username/code/src/git.company.com/username/my-work-store.

Proposal

I personally switched over to gopass some time ago, as I found it more intuitive than pass when I began managing passwords for clients and other organizations. It would be great to have a credential helper that interfaced with gopass. I currently utilize pass only because it is the only available credential helper that appeals to me; I would like to remove this dependency and utilize the password manager I use for everything else.

Note: I'm drafting this issue here, but fully plan on contributing to the project and writing this helper myself within the next few weeks. If this would be ill received, please advise.

Start by reviewing the repository's existing credential helpers and the gopass configuration and command behavior described in this issue. Define the helper's expected interface and verify that it supports both the root store and mounted stores; the issue does not name specific files or tests for validation.