Add support for vim keymap and emulation

[maranix]

Addresses #2902 by adding back support for vim-mode, which was removed from the new version.

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[johnpryan]

The code looks good to me, but I'm wondering if it's possible to make the esc key return the editor back to "normal" mode.

[maranix]

@johnpryan I will look into it and open a PR if that can be done.