Support --in-place generation in Emacs

[Lorenzo Bolla]

Hi!
I use password-store.el to manage my passwords.
I noticed that `password-store-generate` does not support "in-place"
editing: https://git.zx2c4.com/password-store/tree/contrib/emacs/password-store.el#n117
Is this the right place to submit a feature request? ;-)

Thanks!
L.