* Default to showing vi mode prompt (if the current bind mode is vi). Allow overriding by setting it `theme_display_vi` to `no`.
* Remove `theme_display_vi_hide_mode`, because I don't like the prompt jumping around when changing modes.
* Override `fish_mode_prompt` because we're already handling it inside `fish_prompt`. Add a note so future code archeologists can figure out how to override.
* Add support for `replace-one` bind mode.
Fixes#65
