* 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
Add ability to ignore error on missing pass file to allow processing the
output further via another filters (mainly the default filter) without
updating the pass file itself.
It also contains the option to create the pass file, like the option
create=true does.
Finally, it also allows to issue a warning only, if the pass file is not
found.
