Integrate readline vim mode

Thanks for this awesome package.

I'd be even more happy if the existing vim mode of the underlying readline library would be supported. It got integrated in 2015: https://github.com/chzyer/readline/commit/79d1bf27b4a097f08e01f85485f11086a78eacf6.

I'm not a go programmer and don't know how to integrate the support. Maybe it is already integrated somehow, but not documented?