Support for kdbx

[ecstrema]

As a pass directory is the standard for GNU pass, kdbx is the standard for keepssxc and is used quite widely around.

However, the existing iOS apps supporting kdbx are not native, and as such have more difficulties with native extensions and integrate cleanly.

Since kdbx is simpler than a git repo, integrating it should be simpler also.

Also, as far as I know, no other app has support for both kdbx and the pass directory, so this would make this one unique in its kind.

Thanks for the great password manager!

I should get a Mac in the next months. I'll see if I have time to contribute to this here.