Support external password input

I was recently talking to @zx2c4 about switching pass to sq. He asked if we supported something like GnuPG's pinentry, and explained that many users run pass via a launcher and not in a terminal. For instance KDE has a component called KRunner. It is launched via alt+space and allows the user to execute a single command. The output is not shown. Thus, prompting via the terminal is not possible.

We could add a graphical password prompt to sq, and use it if sq does not have a tty, or if it is explicitly selected via a command-line flag or configuration option. This approach has the disadvantage that the integration with the desktop environment will often be suboptimal, because we'll have to decide on one toolkit. And even then, one toolkit is a huge dependency. Alternative, we could do something like pinetry, and use an external program to prompt for the password.

I think that @pbrunschwig also wanted similar functionality for Enigmail.

Thoughts? Other ideas?