[pass] Add optional subdirectory to grep command

[dllud]

Hi everyone,

"pass grep" should have an optional subdirectory argument allowing us to
narrow our searches. A grep on large password tree takes a long long
time due to all the calls to GnuPG.

The new command structure would be like this:
pass grep search-string subfolder

Hope you can give it a go.

Thanks for building password-store and releasing it as Free Software!

Regards.