add support for keepassxc

[ayushnix]

It looks like keepassxc provides a CLI interface and there are a few dmenu wrappers that use it although they also support adding, editing, and generating passwords. I'm not sure if I want to support those features.

Apparently, there's also something called the secret service API on Linux to retrieve passwords from an application that supports this and keepassxc seems to do that. If this is agnostic towards desktop environments and window managers, makes sense to support this as well.