Feature request: configure gpg program

[Thomas Kerin]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA512

I've used pass for a long while, but I've run into something I feel 
warrants a PR

Recently I moved to QubesOS and started using their split-gpg service to 
proxy requests to `/usr/bin/gpg` to a separate VM which holds the 
private keys. Unfortunately for me, pass has a hard coded section where 
it detects which gpg version is installed. I would like to avoid this 
entirely, and point it to the qubes gpg client: 
/usr/bin/qubes-gpg-client-wrapper.

Until recently I just copied pass and modified the line by hand, but 
seeing how configurable it already is, I'd like to add it as an 
environment variable: PASSWORD_STORE_GPG_PROGRAM

As such, I wish to submit this commit for review by the password-store team:

https://github.com/afk11/password-store/commit/54874de5ba1842a4a97cd88f6d5504141b6a7815

If PASSWORD_STORE_GPG_PROGRAM is set, pass will completely defer to it. 
Otherwise, it uses the existing gpg program detection.

I'm pretty sure this would make a few Qubes users happy - a few other 
applications using gpg (eg, thunderbird, or git) also provide ways of 
selecting which gpg program to use.

Best regards,

Thomas Kerin
-----BEGIN PGP SIGNATURE-----
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=1d+e
-----END PGP SIGNATURE-----