pass list

[Pierre Prinetti]

I want to use pass in a system menu that isn't dmenu.

When I look at passmenu[1], I see that most of its complexity is about
printing a list of password names.

If pass was able to print that list, I wouldn't need any plugin and I
could wire that up in my compositor configuration.

Example tree (current behaviour):

```
$ pass list
Password Store
├── one
├── two
├── folder
│   └── three
└── four
```

Example flat list (proposed behaviour):

```
$ pass list -q
one
two
folder/three
four
```

I propose that pass prints a flat list (instead of a tree) when:
* `list -q`; and/or
* `list --flat`; and/or
* `list` is piped; and/or
* other ideas?

I am willing to write a patch if we find consensus.

-Pierre

[1]: https://git.zx2c4.com/password-store/tree/contrib/dmenu/passmenu