Add support for gopass

[SevereOverfl0w]

Gopass is a pass-compatible implementation which adds new features. https://github.com/justwatchcom/gopass

Basic support is in allowing selection of gopass as command to execute.

Full support is in using gopass ls --flat in order to list passwords, which will then cover a particular gopass feature called "mounts"

[maximbaz]

Hey @SevereOverfl0w, browserpass is not using pass as a binary for a number of reasons, it is using gpg directly to decrypt the files. Since gopass is backwards-compatible, I suppose basic support should already be in place.

Now supporting "mounts" is an interesting case, it will definitely not work out of the box as this is a different folder in the file system. I would like to hear other's ideas on how to best add support for these.

You can still access one password-store at a time by defining $PASSWORD_STORE_DIR environment variable, having access to multiple stores (mounts) at the same time is what is being questioned here.

UPDATE: gopass mounts feature request is already requested in #77, so let's ignore mounts in this ticket.
Is there anything in particular besides mounts that you would like to be supported from gopass?

[maximbaz]

Closing as duplicate of #77, if you have additional requests for specific gopass features, please create separate issues for them.