[split] Move simplevi theme to oh-my-fish/theme-simplevi

https://github.com/oh-my-fish/theme-simplevi

themes/simplevi/README.md

-fish_prompt-simplevi
-====================
-
-The stock prompt with the vi indicator appended.
-It is meant to be used with the `vi-mode` plugin.
-
-Screenshots:
-
-![normal mode](https://raw.github.com/syl20bnr/fish_prompt-simplevi/master/prompt_fish-simplevi-n.png)
-
-![insert mode](https://raw.github.com/syl20bnr/fish_prompt-simplevi/master/prompt_fish-simplevi-i.png)
-

themes/simplevi/fish_prompt.fish

-# name: simplevi
-function fish_prompt -d "Write out the prompt"
-  printf '%s@%s%s%s%s [%s]> ' (whoami) (hostname|cut -d . -f 1) (set_color $fish_color_cwd) (prompt_pwd) (set_color normal) $vi_mode
-end