* passwordstore: Make compatible with shims, add backend config
This allows using the passwordstore plugin with scripts that wrap other
password managers. Also adds an explicit configuration (`backend` in
`ini` and `passwordstore_backend` in `vars`) to set the backend to `pass`
(the default) or `gopass`, which allows using gopass as the backend
without the need of a wrapper script. Please be aware that gopass
support is currently limited, but will work for basic operations.
Includes integrations tests.
Resolves#4766
* Apply suggestions from code review
